From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id F2377C433EF for ; Wed, 10 Nov 2021 10:09:10 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 2FBAB61107 for ; Wed, 10 Nov 2021 10:09:10 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 2FBAB61107 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=nongnu.org Received: from localhost ([::1]:51882 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1mkkXU-0002Hr-Tv for qemu-devel@archiver.kernel.org; Wed, 10 Nov 2021 05:09:08 -0500 Received: from eggs.gnu.org ([209.51.188.92]:59136) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mkkW1-0001Wc-UG for qemu-devel@nongnu.org; Wed, 10 Nov 2021 05:07:38 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:32160) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mkkVw-00021Y-8a for qemu-devel@nongnu.org; Wed, 10 Nov 2021 05:07:36 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1636538851; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=hap6m+lNi64CHP69ykTtAsmX2qOGLis8eTfxstHKV6o=; b=gR/WWaXB7bZ116B9a+ZyhcOef0saTiK4U4c7woMdxLuhb5BChAsHWXfGBlLq6YFTjyPQnL fXFVKQsXbNrCXu2pNZtt4jxqiiAyzmU8RYCXghShHtoop8+5p9Lb032EfUij7qZJADNEjc RewIqOlZLL/OJTZTynAbCkq/lz9w0Pw= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-426-4AzMKL4ZNJqRVLb23iiDCQ-1; Wed, 10 Nov 2021 05:07:27 -0500 X-MC-Unique: 4AzMKL4ZNJqRVLb23iiDCQ-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 5FDDC1926DD3; Wed, 10 Nov 2021 10:07:24 +0000 (UTC) Received: from localhost (unknown [10.39.193.165]) by smtp.corp.redhat.com (Postfix) with ESMTP id 69DCF10016F2; Wed, 10 Nov 2021 10:05:51 +0000 (UTC) Date: Wed, 10 Nov 2021 10:05:50 +0000 From: Stefan Hajnoczi To: Christian Schoenebeck Subject: Re: [PATCH v2 0/3] virtio: increase VIRTQUEUE_MAX_SIZE to 32k Message-ID: References: <10421947.PG2v2feK4y@silver> <25571471.tMsSMU6axZ@silver> MIME-Version: 1.0 In-Reply-To: <25571471.tMsSMU6axZ@silver> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=stefanha@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="eestfZp/R6WSRbB0" Content-Disposition: inline Received-SPF: pass client-ip=170.10.133.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.699, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Laurent Vivier , qemu-block@nongnu.org, "Michael S. Tsirkin" , Jason Wang , Amit Shah , David Hildenbrand , qemu-devel@nongnu.org, Greg Kurz , virtio-fs@redhat.com, Eric Auger , Hanna Reitz , "Gonglei \(Arei\)" , Gerd Hoffmann , =?iso-8859-1?Q?Marc-Andr=E9?= Lureau , Paolo Bonzini , Fam Zheng , Raphael Norwitz , "Dr. David Alan Gilbert" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" --eestfZp/R6WSRbB0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Nov 09, 2021 at 02:09:59PM +0100, Christian Schoenebeck wrote: > On Dienstag, 9. November 2021 11:56:35 CET Stefan Hajnoczi wrote: > > On Thu, Nov 04, 2021 at 03:41:23PM +0100, Christian Schoenebeck wrote: > > > On Mittwoch, 3. November 2021 12:33:33 CET Stefan Hajnoczi wrote: > > > > On Mon, Nov 01, 2021 at 09:29:26PM +0100, Christian Schoenebeck wro= te: > > > > > On Donnerstag, 28. Oktober 2021 11:00:48 CET Stefan Hajnoczi wrot= e: > > > > > > On Mon, Oct 25, 2021 at 05:03:25PM +0200, Christian Schoenebeck= =20 > wrote: > > > > > > > On Montag, 25. Oktober 2021 12:30:41 CEST Stefan Hajnoczi wro= te: > > > > > > > > On Thu, Oct 21, 2021 at 05:39:28PM +0200, Christian Schoene= beck=20 > wrote: > > > > > > > > > On Freitag, 8. Oktober 2021 18:08:48 CEST Christian=20 > Schoenebeck wrote: > > > > > > > > > > On Freitag, 8. Oktober 2021 16:24:42 CEST Christian=20 > Schoenebeck wrote: > > > > > > > > > > > On Freitag, 8. Oktober 2021 09:25:33 CEST Greg Kurz w= rote: > > > > > > > > > > > > On Thu, 7 Oct 2021 16:42:49 +0100 > > > > > > > > > > > >=20 > > > > > > > > > > > > Stefan Hajnoczi wrote: > > > > > > > > > > > > > On Thu, Oct 07, 2021 at 02:51:55PM +0200, Christi= an=20 > Schoenebeck wrote: > > > > > > > > > > > > > > On Donnerstag, 7. Oktober 2021 07:23:59 CEST St= efan=20 > Hajnoczi wrote: > > > > > > > > > > > > > > > On Mon, Oct 04, 2021 at 09:38:00PM +0200, > > > > > > > > > > > > > > > Christian > > > > > > > > > > > > > > > Schoenebeck > > > > > > > > > >=20 > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > At the moment the maximum transfer size wit= h > > > > > > > > > > > > > > > > virtio > > > > > > > > > > > > > > > > is > > > > > > > > > > > > > > > > limited > > > > > > > > > > > > > > > > to > > > > > > > > > > > > > > > > 4M > > > > > > > > > > > > > > > > (1024 * PAGE_SIZE). This series raises this > > > > > > > > > > > > > > > > limit to > > > > > > > > > > > > > > > > its > > > > > > > > > > > > > > > > maximum > > > > > > > > > > > > > > > > theoretical possible transfer size of 128M = (32k > > > > > > > > > > > > > > > > pages) > > > > > > > > > > > > > > > > according > > > > > > > > > > > > > > > > to > > > > > > > > > > > > > > > > the > > > > > > > > > > > > > > > > virtio specs: > > > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > > > https://docs.oasis-open.org/virtio/virtio/v= 1.1/c > > > > > > > > > > > > > > > > s01/ > > > > > > > > > > > > > > > > virt > > > > > > > > > > > > > > > > io-v > > > > > > > > > > > > > > > > 1.1- > > > > > > > > > > > > > > > > cs > > > > > > > > > > > > > > > > 01 > > > > > > > > > > > > > > > > .html# > > > > > > > > > > > > > > > > x1-240006 > > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > > Hi Christian, > > > > > > > > > > > >=20 > > > > > > > > > > > > > > > I took a quick look at the code: > > > > > > > > > > > > Hi, > > > > > > > > > > > >=20 > > > > > > > > > > > > Thanks Stefan for sharing virtio expertise and help= ing > > > > > > > > > > > > Christian > > > > > > > > > > > > ! > > > > > > > > > > > >=20 > > > > > > > > > > > > > > > - The Linux 9p driver restricts descriptor ch= ains > > > > > > > > > > > > > > > to > > > > > > > > > > > > > > > 128 > > > > > > > > > > > > > > > elements > > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > > (net/9p/trans_virtio.c:VIRTQUEUE_NUM) > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > Yes, that's the limitation that I am about to r= emove > > > > > > > > > > > > > > (WIP); > > > > > > > > > > > > > > current > > > > > > > > > > > > > > kernel > > > > > > > > > > > > > > patches: > > > > > > > > > > > > > > https://lore.kernel.org/netdev/cover.1632327421= .git. > > > > > > > > > > > > > > linu > > > > > > > > > > > > > > x_os > > > > > > > > > > > > > > s@cr > > > > > > > > > > > > > > udeb > > > > > > > > > > > > > > yt > > > > > > > > > > > > > > e. > > > > > > > > > > > > > > com/> > > > > > > > > > > > > >=20 > > > > > > > > > > > > > I haven't read the patches yet but I'm concerned = that > > > > > > > > > > > > > today > > > > > > > > > > > > > the > > > > > > > > > > > > > driver > > > > > > > > > > > > > is pretty well-behaved and this new patch series > > > > > > > > > > > > > introduces a > > > > > > > > > > > > > spec > > > > > > > > > > > > > violation. Not fixing existing spec violations is > > > > > > > > > > > > > okay, > > > > > > > > > > > > > but > > > > > > > > > > > > > adding > > > > > > > > > > > > > new > > > > > > > > > > > > > ones is a red flag. I think we need to figure out= a > > > > > > > > > > > > > clean > > > > > > > > > > > > > solution. > > > > > > > > > > >=20 > > > > > > > > > > > Nobody has reviewed the kernel patches yet. My main > > > > > > > > > > > concern > > > > > > > > > > > therefore > > > > > > > > > > > actually is that the kernel patches are already too > > > > > > > > > > > complex, > > > > > > > > > > > because > > > > > > > > > > > the > > > > > > > > > > > current situation is that only Dominique is handling = 9p > > > > > > > > > > > patches on > > > > > > > > > > > kernel > > > > > > > > > > > side, and he barely has time for 9p anymore. > > > > > > > > > > >=20 > > > > > > > > > > > Another reason for me to catch up on reading current > > > > > > > > > > > kernel > > > > > > > > > > > code > > > > > > > > > > > and > > > > > > > > > > > stepping in as reviewer of 9p on kernel side ASAP, > > > > > > > > > > > independent > > > > > > > > > > > of > > > > > > > > > > > this > > > > > > > > > > > issue. > > > > > > > > > > >=20 > > > > > > > > > > > As for current kernel patches' complexity: I can cert= ainly > > > > > > > > > > > drop > > > > > > > > > > > patch > > > > > > > > > > > 7 > > > > > > > > > > > entirely as it is probably just overkill. Patch 4 is = then > > > > > > > > > > > the > > > > > > > > > > > biggest > > > > > > > > > > > chunk, I have to see if I can simplify it, and whethe= r it > > > > > > > > > > > would > > > > > > > > > > > make > > > > > > > > > > > sense to squash with patch 3. > > > > > > > > > > >=20 > > > > > > > > > > > > > > > - The QEMU 9pfs code passes iovecs directly t= o > > > > > > > > > > > > > > > preadv(2) > > > > > > > > > > > > > > > and > > > > > > > > > > > > > > > will > > > > > > > > > > > > > > > fail > > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > > with EINVAL when called with more than IOV_= MAX > > > > > > > > > > > > > > > iovecs > > > > > > > > > > > > > > > (hw/9pfs/9p.c:v9fs_read()) > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > Hmm, which makes me wonder why I never encounte= red > > > > > > > > > > > > > > this > > > > > > > > > > > > > > error > > > > > > > > > > > > > > during > > > > > > > > > > > > > > testing. > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > Most people will use the 9p qemu 'local' fs dri= ver > > > > > > > > > > > > > > backend > > > > > > > > > > > > > > in > > > > > > > > > > > > > > practice, > > > > > > > > > > > > > > so > > > > > > > > > > > > > > that v9fs_read() call would translate for most > > > > > > > > > > > > > > people to > > > > > > > > > > > > > > this > > > > > > > > > > > > > > implementation on QEMU side (hw/9p/9p-local.c): > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > static ssize_t local_preadv(FsContext *ctx, > > > > > > > > > > > > > > V9fsFidOpenState > > > > > > > > > > > > > > *fs, > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > const struct iovec = *iov, > > > > > > > > > > > > > > int iovcnt, off_t > > > > > > > > > > > > > > offset) > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > { > > > > > > > > > > > > > > #ifdef CONFIG_PREADV > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > return preadv(fs->fd, iov, iovcnt, offset); > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > #else > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > int err =3D lseek(fs->fd, offset, SEEK_SET)= ; > > > > > > > > > > > > > > if (err =3D=3D -1) { > > > > > > > > > > > > > > =20 > > > > > > > > > > > > > > return err; > > > > > > > > > > > > > > =20 > > > > > > > > > > > > > > } else { > > > > > > > > > > > > > > =20 > > > > > > > > > > > > > > return readv(fs->fd, iov, iovcnt); > > > > > > > > > > > > > > =20 > > > > > > > > > > > > > > } > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > #endif > > > > > > > > > > > > > > } > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > > Unless I misunderstood the code, neither side= can > > > > > > > > > > > > > > > take > > > > > > > > > > > > > > > advantage > > > > > > > > > > > > > > > of > > > > > > > > > > > > > > > the > > > > > > > > > > > > > > > new 32k descriptor chain limit? > > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > > > > > > Stefan > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > I need to check that when I have some more time= . One > > > > > > > > > > > > > > possible > > > > > > > > > > > > > > explanation > > > > > > > > > > > > > > might be that preadv() already has this wrapped= into > > > > > > > > > > > > > > a > > > > > > > > > > > > > > loop > > > > > > > > > > > > > > in > > > > > > > > > > > > > > its > > > > > > > > > > > > > > implementation to circumvent a limit like IOV_M= AX. > > > > > > > > > > > > > > It > > > > > > > > > > > > > > might > > > > > > > > > > > > > > be > > > > > > > > > > > > > > another > > > > > > > > > > > > > > "it > > > > > > > > > > > > > > works, but not portable" issue, but not sure. > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > There are still a bunch of other issues I have = to > > > > > > > > > > > > > > resolve. > > > > > > > > > > > > > > If > > > > > > > > > > > > > > you > > > > > > > > > > > > > > look > > > > > > > > > > > > > > at > > > > > > > > > > > > > > net/9p/client.c on kernel side, you'll notice t= hat > > > > > > > > > > > > > > it > > > > > > > > > > > > > > basically > > > > > > > > > > > > > > does > > > > > > > > > > > > > > this ATM> > > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > kmalloc(msize); > > > > > > > > > > > >=20 > > > > > > > > > > > > Note that this is done twice : once for the T messa= ge > > > > > > > > > > > > (client > > > > > > > > > > > > request) > > > > > > > > > > > > and > > > > > > > > > > > > once for the R message (server answer). The 9p driv= er > > > > > > > > > > > > could > > > > > > > > > > > > adjust > > > > > > > > > > > > the > > > > > > > > > > > > size > > > > > > > > > > > > of the T message to what's really needed instead of > > > > > > > > > > > > allocating > > > > > > > > > > > > the > > > > > > > > > > > > full > > > > > > > > > > > > msize. R message size is not known though. > > > > > > > > > > >=20 > > > > > > > > > > > Would it make sense adding a second virtio ring, dedi= cated > > > > > > > > > > > to > > > > > > > > > > > server > > > > > > > > > > > responses to solve this? IIRC 9p server already calcu= lates > > > > > > > > > > > appropriate > > > > > > > > > > > exact sizes for each response type. So server could j= ust > > > > > > > > > > > push > > > > > > > > > > > space > > > > > > > > > > > that's > > > > > > > > > > > really needed for its responses. > > > > > > > > > > >=20 > > > > > > > > > > > > > > for every 9p request. So not only does it alloc= ate > > > > > > > > > > > > > > much > > > > > > > > > > > > > > more > > > > > > > > > > > > > > memory > > > > > > > > > > > > > > for > > > > > > > > > > > > > > every request than actually required (i.e. say = 9pfs > > > > > > > > > > > > > > was > > > > > > > > > > > > > > mounted > > > > > > > > > > > > > > with > > > > > > > > > > > > > > msize=3D8M, then a 9p request that actually wou= ld just > > > > > > > > > > > > > > need 1k > > > > > > > > > > > > > > would > > > > > > > > > > > > > > nevertheless allocate 8M), but also it allocate= s > > > > > > > > > > > > > > > PAGE_SIZE, > > > > > > > > > > > > > > which > > > > > > > > > > > > > > obviously may fail at any time.> > > > > > > > > > > > > >=20 > > > > > > > > > > > > > The PAGE_SIZE limitation sounds like a kmalloc() = vs > > > > > > > > > > > > > vmalloc() > > > > > > > > > > > > > situation. > > > > > > > > > > >=20 > > > > > > > > > > > Hu, I didn't even consider vmalloc(). I just tried th= e > > > > > > > > > > > kvmalloc() > > > > > > > > > > > wrapper > > > > > > > > > > > as a quick & dirty test, but it crashed in the same w= ay as > > > > > > > > > > > kmalloc() > > > > > > > > > > > with > > > > > > > > > > > large msize values immediately on mounting: > > > > > > > > > > >=20 > > > > > > > > > > > diff --git a/net/9p/client.c b/net/9p/client.c > > > > > > > > > > > index a75034fa249b..cfe300a4b6ca 100644 > > > > > > > > > > > --- a/net/9p/client.c > > > > > > > > > > > +++ b/net/9p/client.c > > > > > > > > > > > @@ -227,15 +227,18 @@ static int parse_opts(char *opt= s, > > > > > > > > > > > struct > > > > > > > > > > > p9_client > > > > > > > > > > > *clnt) > > > > > > > > > > >=20 > > > > > > > > > > > static int p9_fcall_init(struct p9_client *c, struct > > > > > > > > > > > p9_fcall > > > > > > > > > > > *fc, > > > > > > > > > > > =20 > > > > > > > > > > > int alloc_msize) > > > > > > > > > > > =20 > > > > > > > > > > > { > > > > > > > > > > >=20 > > > > > > > > > > > - if (likely(c->fcall_cache) && alloc_msize =3D= =3D > > > > > > > > > > > c->msize) > > > > > > > > > > > { > > > > > > > > > > > + //if (likely(c->fcall_cache) && alloc_msize = =3D=3D > > > > > > > > > > > c->msize) { > > > > > > > > > > > + if (false) { > > > > > > > > > > >=20 > > > > > > > > > > > fc->sdata =3D > > > > > > > > > > > kmem_cache_alloc(c->fcall_cache, > > > > > > > > > > > GFP_NOFS); > > > > > > > > > > > fc->cache =3D c->fcall_cache; > > > > > > > > > > > =20 > > > > > > > > > > > } else { > > > > > > > > > > >=20 > > > > > > > > > > > - fc->sdata =3D kmalloc(alloc_msize, > > > > > > > > > > > GFP_NOFS); > > > > > > > > > > > + fc->sdata =3D kvmalloc(alloc_msize, > > > > > > > > > > > GFP_NOFS); > > > > > > > > > >=20 > > > > > > > > > > Ok, GFP_NOFS -> GFP_KERNEL did the trick. > > > > > > > > > >=20 > > > > > > > > > > Now I get: > > > > > > > > > > virtio: bogus descriptor or out of resources > > > > > > > > > >=20 > > > > > > > > > > So, still some work ahead on both ends. > > > > > > > > >=20 > > > > > > > > > Few hacks later (only changes on 9p client side) I got th= is > > > > > > > > > running > > > > > > > > > stable > > > > > > > > > now. The reason for the virtio error above was that kvmal= loc() > > > > > > > > > returns > > > > > > > > > a > > > > > > > > > non-logical kernel address for any kvmalloc(>4M), i.e. an > > > > > > > > > address > > > > > > > > > that > > > > > > > > > is > > > > > > > > > inaccessible from host side, hence that "bogus descriptor= " > > > > > > > > > message > > > > > > > > > by > > > > > > > > > QEMU. > > > > > > > > > So I had to split those linear 9p client buffers into spa= rse > > > > > > > > > ones > > > > > > > > > (set > > > > > > > > > of > > > > > > > > > individual pages). > > > > > > > > >=20 > > > > > > > > > I tested this for some days with various virtio transmiss= ion > > > > > > > > > sizes > > > > > > > > > and > > > > > > > > > it > > > > > > > > > works as expected up to 128 MB (more precisely: 128 MB re= ad > > > > > > > > > space > > > > > > > > > + > > > > > > > > > 128 MB > > > > > > > > > write space per virtio round trip message). > > > > > > > > >=20 > > > > > > > > > I did not encounter a show stopper for large virtio > > > > > > > > > transmission > > > > > > > > > sizes > > > > > > > > > (4 MB ... 128 MB) on virtio level, neither as a result of > > > > > > > > > testing, > > > > > > > > > nor > > > > > > > > > after reviewing the existing code. > > > > > > > > >=20 > > > > > > > > > About IOV_MAX: that's apparently not an issue on virtio l= evel. > > > > > > > > > Most of > > > > > > > > > the > > > > > > > > > iovec code, both on Linux kernel side and on QEMU side do= not > > > > > > > > > have > > > > > > > > > this > > > > > > > > > limitation. It is apparently however indeed a limitation = for > > > > > > > > > userland > > > > > > > > > apps > > > > > > > > > calling the Linux kernel's syscalls yet. > > > > > > > > >=20 > > > > > > > > > Stefan, as it stands now, I am even more convinced that t= he > > > > > > > > > upper > > > > > > > > > virtio > > > > > > > > > transmission size limit should not be squeezed into the q= ueue > > > > > > > > > size > > > > > > > > > argument of virtio_add_queue(). Not because of the previo= us > > > > > > > > > argument > > > > > > > > > that > > > > > > > > > it would waste space (~1MB), but rather because they are = two > > > > > > > > > different > > > > > > > > > things. To outline this, just a quick recap of what happe= ns > > > > > > > > > exactly > > > > > > > > > when > > > > > > > > > a bulk message is pushed over the virtio wire (assuming v= irtio > > > > > > > > > "split" > > > > > > > > > layout here): > > > > > > > > >=20 > > > > > > > > > ---------- [recap-start] ---------- > > > > > > > > >=20 > > > > > > > > > For each bulk message sent guest <-> host, exactly *one* = of > > > > > > > > > the > > > > > > > > > pre-allocated descriptors is taken and placed (subsequent= ly) > > > > > > > > > into > > > > > > > > > exactly > > > > > > > > > *one* position of the two available/used ring buffers. Th= e > > > > > > > > > actual > > > > > > > > > descriptor table though, containing all the DMA addresses= of > > > > > > > > > the > > > > > > > > > message > > > > > > > > > bulk data, is allocated just in time for each round trip > > > > > > > > > message. > > > > > > > > > Say, > > > > > > > > > it > > > > > > > > > is the first message sent, it yields in the following > > > > > > > > > structure: > > > > > > > > >=20 > > > > > > > > > Ring Buffer Descriptor Table Bulk Data Pages > > > > > > > > >=20 > > > > > > > > > +-+ +-+ +-----------------+ > > > > > > > > > =20 > > > > > > > > > |D|------------->|d|---------->| Bulk data block | > > > > > > > > > =20 > > > > > > > > > +-+ |d|--------+ +-----------------+ > > > > > > > > > =20 > > > > > > > > > | | |d|------+ | > > > > > > > > > =20 > > > > > > > > > +-+ . | | +-----------------+ > > > > > > > > > =20 > > > > > > > > > | | . | +->| Bulk data block | > > > > > > > > > =20 > > > > > > > > > . . | +-----------------+ > > > > > > > > > . |d|-+ | > > > > > > > > > . +-+ | | +-----------------+ > > > > > > > > > =20 > > > > > > > > > | | | +--->| Bulk data block | > > > > > > > > > =20 > > > > > > > > > +-+ | +-----------------+ > > > > > > > > > =20 > > > > > > > > > | | | . > > > > > > > > > =20 > > > > > > > > > +-+ | . > > > > > > > > > =20 > > > > > > > > > | . > > > > > > > > > | =20 > > > > > > > > > | +-----------------+ > > > > > > > > > =20 > > > > > > > > > +-------->| Bulk data block | > > > > > > > > > =20 > > > > > > > > > +-----------------+ > > > > > > > > >=20 > > > > > > > > > Legend: > > > > > > > > > D: pre-allocated descriptor > > > > > > > > > d: just in time allocated descriptor > > > > > > > > > -->: memory pointer (DMA) > > > > > > > > >=20 > > > > > > > > > The bulk data blocks are allocated by the respective devi= ce > > > > > > > > > driver > > > > > > > > > above > > > > > > > > > virtio subsystem level (guest side). > > > > > > > > >=20 > > > > > > > > > There are exactly as many descriptors pre-allocated (D) a= s the > > > > > > > > > size of > > > > > > > > > a > > > > > > > > > ring buffer. > > > > > > > > >=20 > > > > > > > > > A "descriptor" is more or less just a chainable DMA memor= y > > > > > > > > > pointer; > > > > > > > > > defined > > > > > > > > > as: > > > > > > > > >=20 > > > > > > > > > /* Virtio ring descriptors: 16 bytes. These can chain > > > > > > > > > together > > > > > > > > > via > > > > > > > > > "next". */ struct vring_desc { > > > > > > > > >=20 > > > > > > > > > =09/* Address (guest-physical). */ > > > > > > > > > =09__virtio64 addr; > > > > > > > > > =09/* Length. */ > > > > > > > > > =09__virtio32 len; > > > > > > > > > =09/* The flags as indicated above. */ > > > > > > > > > =09__virtio16 flags; > > > > > > > > > =09/* We chain unused descriptors via this, too */ > > > > > > > > > =09__virtio16 next; > > > > > > > > >=20 > > > > > > > > > }; > > > > > > > > >=20 > > > > > > > > > There are 2 ring buffers; the "available" ring buffer is = for > > > > > > > > > sending a > > > > > > > > > message guest->host (which will transmit DMA addresses of > > > > > > > > > guest > > > > > > > > > allocated > > > > > > > > > bulk data blocks that are used for data sent to device, a= nd > > > > > > > > > separate > > > > > > > > > guest allocated bulk data blocks that will be used by hos= t > > > > > > > > > side to > > > > > > > > > place > > > > > > > > > its response bulk data), and the "used" ring buffer is fo= r > > > > > > > > > sending > > > > > > > > > host->guest to let guest know about host's response and t= hat > > > > > > > > > it > > > > > > > > > could > > > > > > > > > now > > > > > > > > > safely consume and then deallocate the bulk data blocks > > > > > > > > > subsequently. > > > > > > > > >=20 > > > > > > > > > ---------- [recap-end] ---------- > > > > > > > > >=20 > > > > > > > > > So the "queue size" actually defines the ringbuffer size.= It > > > > > > > > > does > > > > > > > > > not > > > > > > > > > define the maximum amount of descriptors. The "queue size= " > > > > > > > > > rather > > > > > > > > > defines > > > > > > > > > how many pending messages can be pushed into either one > > > > > > > > > ringbuffer > > > > > > > > > before > > > > > > > > > the other side would need to wait until the counter side = would > > > > > > > > > step up > > > > > > > > > (i.e. ring buffer full). > > > > > > > > >=20 > > > > > > > > > The maximum amount of descriptors (what VIRTQUEUE_MAX_SIZ= E > > > > > > > > > actually > > > > > > > > > is) > > > > > > > > > OTOH defines the max. bulk data size that could be transm= itted > > > > > > > > > with > > > > > > > > > each > > > > > > > > > virtio round trip message. > > > > > > > > >=20 > > > > > > > > > And in fact, 9p currently handles the virtio "queue size"= as > > > > > > > > > directly > > > > > > > > > associative with its maximum amount of active 9p requests= the > > > > > > > > > server > > > > > > > > > could > > > > > > > > >=20 > > > > > > > > > handle simultaniously: > > > > > > > > > hw/9pfs/9p.h:#define MAX_REQ 128 > > > > > > > > > hw/9pfs/9p.h: V9fsPDU pdus[MAX_REQ]; > > > > > > > > > hw/9pfs/virtio-9p-device.c: v->vq =3D > > > > > > > > > virtio_add_queue(vdev, > > > > > > > > > MAX_REQ, > > > > > > > > > =20 > > > > > > > > > handle_9p_output); > > > > > > > > >=20 > > > > > > > > > So if I would change it like this, just for the purpose t= o > > > > > > > > > increase > > > > > > > > > the > > > > > > > > > max. virtio transmission size: > > > > > > > > >=20 > > > > > > > > > --- a/hw/9pfs/virtio-9p-device.c > > > > > > > > > +++ b/hw/9pfs/virtio-9p-device.c > > > > > > > > > @@ -218,7 +218,7 @@ static void > > > > > > > > > virtio_9p_device_realize(DeviceState > > > > > > > > > *dev, > > > > > > > > > Error **errp)> > > > > > > > > >=20 > > > > > > > > > v->config_size =3D sizeof(struct virtio_9p_config) + > > > > > > > > > strlen(s->fsconf.tag); > > > > > > > > > virtio_init(vdev, "virtio-9p", VIRTIO_ID_9P, > > > > > > > > > v->config_size, > > > > > > > > > =20 > > > > > > > > > VIRTQUEUE_MAX_SIZE); > > > > > > > > >=20 > > > > > > > > > - v->vq =3D virtio_add_queue(vdev, MAX_REQ, > > > > > > > > > handle_9p_output); > > > > > > > > > + v->vq =3D virtio_add_queue(vdev, 32*1024, > > > > > > > > > handle_9p_output); > > > > > > > > >=20 > > > > > > > > > } > > > > > > > > >=20 > > > > > > > > > Then it would require additional synchronization code on = both > > > > > > > > > ends > > > > > > > > > and > > > > > > > > > therefore unnecessary complexity, because it would now be > > > > > > > > > possible > > > > > > > > > that > > > > > > > > > more requests are pushed into the ringbuffer than server = could > > > > > > > > > handle. > > > > > > > > >=20 > > > > > > > > > There is one potential issue though that probably did jus= tify > > > > > > > > > the > > > > > > > > > "don't > > > > > > > > > exceed the queue size" rule: > > > > > > > > >=20 > > > > > > > > > ATM the descriptor table is allocated (just in time) as *= one* > > > > > > > > > continuous > > > > > > > > > buffer via kmalloc_array(): > > > > > > > > > https://github.com/torvalds/linux/blob/2f111a6fd5b5297b4e= 92f53 > > > > > > > > > 798c > > > > > > > > > a086 > > > > > > > > > f7c7 > > > > > > > > > d33a4/drivers/virtio/virtio_ring.c#L440 > > > > > > > > >=20 > > > > > > > > > So assuming transmission size of 2 * 128 MB that > > > > > > > > > kmalloc_array() > > > > > > > > > call > > > > > > > > > would > > > > > > > > > yield in kmalloc(1M) and the latter might fail if guest h= ad > > > > > > > > > highly > > > > > > > > > fragmented physical memory. For such kind of error case t= here > > > > > > > > > is > > > > > > > > > currently a fallback path in virtqueue_add_split() that w= ould > > > > > > > > > then > > > > > > > > > use > > > > > > > > > the required amount of pre-allocated descriptors instead: > > > > > > > > > https://github.com/torvalds/linux/blob/2f111a6fd5b5297b4e= 92f53 > > > > > > > > > 798c > > > > > > > > > a086 > > > > > > > > > f7c7 > > > > > > > > > d33a4/drivers/virtio/virtio_ring.c#L525 > > > > > > > > >=20 > > > > > > > > > That fallback recovery path would no longer be viable if = the > > > > > > > > > queue > > > > > > > > > size > > > > > > > > > was > > > > > > > > > exceeded. There would be alternatives though, e.g. by all= owing > > > > > > > > > to > > > > > > > > > chain > > > > > > > > > indirect descriptor tables (currently prohibited by the v= irtio > > > > > > > > > specs). > > > > > > > >=20 > > > > > > > > Making the maximum number of descriptors independent of the > > > > > > > > queue > > > > > > > > size > > > > > > > > requires a change to the VIRTIO spec since the two values a= re > > > > > > > > currently > > > > > > > > explicitly tied together by the spec. > > > > > > >=20 > > > > > > > Yes, that's what the virtio specs say. But they don't say why= , nor > > > > > > > did > > > > > > > I > > > > > > > hear a reason in this dicussion. > > > > > > >=20 > > > > > > > That's why I invested time reviewing current virtio implement= ation > > > > > > > and > > > > > > > specs, as well as actually testing exceeding that limit. And = as I > > > > > > > outlined in detail in my previous email, I only found one > > > > > > > theoretical > > > > > > > issue that could be addressed though. > > > > > >=20 > > > > > > I agree that there is a limitation in the VIRTIO spec, but viol= ating > > > > > > the > > > > > > spec isn't an acceptable solution: > > > > > >=20 > > > > > > 1. QEMU and Linux aren't the only components that implement VIR= TIO. > > > > > > You > > > > > >=20 > > > > > > cannot make assumptions about their implementations because = it > > > > > > may > > > > > > break spec-compliant implementations that you haven't looked= at. > > > > > > =20 > > > > > > Your patches weren't able to increase Queue Size because som= e > > > > > > device > > > > > > implementations break when descriptor chains are too long. T= his > > > > > > shows > > > > > > there is a practical issue even in QEMU. > > > > > >=20 > > > > > > 2. The specific spec violation that we discussed creates the pr= oblem > > > > > >=20 > > > > > > that drivers can no longer determine the maximum description > > > > > > chain > > > > > > length. This in turn will lead to more implementation-specif= ic > > > > > > assumptions being baked into drivers and cause problems with > > > > > > interoperability and future changes. > > > > > >=20 > > > > > > The spec needs to be extended instead. I included an idea for h= ow to > > > > > > do > > > > > > that below. > > > > >=20 > > > > > Sure, I just wanted to see if there was a non-neglectable "hard" = show > > > > > stopper per se that I probably haven't seen yet. I have not quest= ioned > > > > > aiming a clean solution. > > > > >=20 > > > > > Thanks for the clarification! > > > > >=20 > > > > > > > > Before doing that, are there benchmark results showing that= 1 MB > > > > > > > > vs > > > > > > > > 128 > > > > > > > > MB produces a performance improvement? I'm asking because i= f > > > > > > > > performance > > > > > > > > with 1 MB is good then you can probably do that without hav= ing > > > > > > > > to > > > > > > > > change > > > > > > > > VIRTIO and also because it's counter-intuitive that 9p need= s 128 > > > > > > > > MB > > > > > > > > for > > > > > > > > good performance when it's ultimately implemented on top of= disk > > > > > > > > and > > > > > > > > network I/O that have lower size limits. > > > > > > >=20 > > > > > > > First some numbers, linear reading a 12 GB file: > > > > > > >=20 > > > > > > > msize average notes > > > > > > >=20 > > > > > > > 8 kB 52.0 MB/s default msize of Linux kernel > > > > > > 128 kB 624.8 MB/s default msize of Linux kernel >=3Dv5.15 > > > > > > > 512 kB 1961 MB/s current max. msize with any Linux kerne= l > > > > > > > <=3Dv5.15 > > > > > > > 1 MB 2551 MB/s this msize would already violate virtio > > > > > > > specs > > > > > > > 2 MB 2521 MB/s this msize would already violate virtio > > > > > > > specs > > > > > > > 4 MB 2628 MB/s planned max. msize of my current kernel > > > > > > > patches > > > > > > > [1] > > > > > >=20 > > > > > > How many descriptors are used? 4 MB can be covered by a single > > > > > > descriptor if the data is physically contiguous in memory, so t= his > > > > > > data > > > > > > doesn't demonstrate a need for more descriptors. > > > > >=20 > > > > > No, in the last couple years there was apparently no kernel versi= on > > > > > that > > > > > used just one descriptor, nor did my benchmarked version. Even th= ough > > > > > the > > > > > Linux 9p client uses (yet) simple linear buffers (contiguous phys= ical > > > > > memory) on 9p client level, these are however split into PAGE_SIZ= E > > > > > chunks > > > > > by function pack_sg_list() [1] before being fed to virtio level: > > > > >=20 > > > > > static unsigned int rest_of_page(void *data) > > > > > { > > > > >=20 > > > > > =09return PAGE_SIZE - offset_in_page(data); > > > > >=20 > > > > > } > > > > > ... > > > > > static int pack_sg_list(struct scatterlist *sg, int start, > > > > >=20 > > > > > =09=09=09int limit, char *data, int count) > > > > >=20 > > > > > { > > > > >=20 > > > > > =09int s; > > > > > =09int index =3D start; > > > > > =09 > > > > > =09while (count) { > > > > > =09 > > > > > =09=09s =3D rest_of_page(data); > > > > > =09=09... > > > > > =09=09sg_set_buf(&sg[index++], data, s); > > > > > =09=09count -=3D s; > > > > > =09=09data +=3D s; > > > > > =09 > > > > > =09} > > > > > =09... > > > > >=20 > > > > > } > > > > >=20 > > > > > [1] > > > > > https://github.com/torvalds/linux/blob/19901165d90fdca1e57c9baa0d= 5b4c6 > > > > > 3d1 > > > > > 5c476a/net/9p/trans_virtio.c#L171 > > > > >=20 > > > > > So when sending 4MB over virtio wire, it would yield in 1k descri= ptors > > > > > ATM. > > > > >=20 > > > > > I have wondered about this before, but did not question it, becau= se > > > > > due to > > > > > the cross-platform nature I couldn't say for certain whether that= 's > > > > > probably needed somewhere. I mean for the case virtio-PCI I know = for > > > > > sure > > > > > that one descriptor (i.e. >PAGE_SIZE) would be fine, but I don't = know > > > > > if > > > > > that applies to all buses and architectures. > > > >=20 > > > > VIRTIO does not limit descriptor the descriptor len field to PAGE_S= IZE, > > > > so I don't think there is a limit at the VIRTIO level. > > >=20 > > > So you are viewing this purely from virtio specs PoV: in the sense, i= f it > > > is not prohibited by the virtio specs, then it should work. Maybe. > >=20 > > Limitations must be specified either in the 9P protocol or the VIRTIO > > specification. Drivers and devices will not be able to operate correctl= y > > if there are limitations that aren't covered by the specs. > >=20 > > Do you have something in mind that isn't covered by the specs? >=20 > Not sure whether that's something that should be specified by the virtio= =20 > specs, probably not. I simply do not know if there was any bus or archite= cture=20 > that would have a limitation for max. size for a memory block passed per = one=20 > DMA address. Host-side limitations like that can exist. For example when a physical storage device on the host has limits that the VIRTIO device does not have. In this case both virtio-scsi and virtio-blk report those limits to the guest so that the guest won't submit requests that the physical device would reject. I guess networking MTU is kind of similar too. What they have in common is that the limit needs to be reported to the guest, typically using a VIRTIO Configuration Space field. It is an explicit limit that is part of the host<->guest interface (VIRTIO spec, SCSI, etc). > > > > If this function coalesces adjacent pages then the descriptor chain > > > > length issues could be reduced. > > > >=20 > > > > > > > But again, this is not just about performance. My conclusion = as > > > > > > > described > > > > > > > in my previous email is that virtio currently squeezes > > > > > > >=20 > > > > > > > =09"max. simultanious amount of bulk messages" > > > > > > >=20 > > > > > > > vs. > > > > > > >=20 > > > > > > > =09"max. bulk data transmission size per bulk messaage" > > > > > > >=20 > > > > > > > into the same configuration parameter, which is IMO inappropr= iate > > > > > > > and > > > > > > > hence > > > > > > > splitting them into 2 separate parameters when creating a que= ue > > > > > > > makes > > > > > > > sense, independent of the performance benchmarks. > > > > > > >=20 > > > > > > > [1] > > > > > > > https://lore.kernel.org/netdev/cover.1632327421.git.linux_oss= @crud > > > > > > > ebyt > > > > > > > e.c > > > > > > > om/ > > > > > >=20 > > > > > > Some devices effectively already have this because the device > > > > > > advertises > > > > > > a maximum number of descriptors via device-specific mechanisms = like > > > > > > the > > > > > > struct virtio_blk_config seg_max field. But today these fields = can > > > > > > only > > > > > > reduce the maximum descriptor chain length because the spec sti= ll > > > > > > limits > > > > > > the length to Queue Size. > > > > > >=20 > > > > > > We can build on this approach to raise the length above Queue S= ize. > > > > > > This > > > > > > approach has the advantage that the maximum number of segments = isn't > > > > > > per > > > > > > device or per virtqueue, it's fine-grained. If the device suppo= rts > > > > > > two > > > > > > requests types then different max descriptor chain limits could= be > > > > > > given > > > > > > for them by introducing two separate configuration space fields= . > > > > > >=20 > > > > > > Here are the corresponding spec changes: > > > > > >=20 > > > > > > 1. A new feature bit called VIRTIO_RING_F_LARGE_INDIRECT_DESC i= s > > > > > > added > > > > > >=20 > > > > > > to indicate that indirect descriptor table size and maximum > > > > > > descriptor chain length are not limited by Queue Size value. > > > > > > (Maybe > > > > > > there still needs to be a limit like 2^15?) > > > > >=20 > > > > > Sounds good to me! > > > > >=20 > > > > > AFAIK it is effectively limited to 2^16 because of vring_desc->ne= xt: > > > > >=20 > > > > > /* Virtio ring descriptors: 16 bytes. These can chain together v= ia > > > > > "next". */ struct vring_desc { > > > > >=20 > > > > > /* Address (guest-physical). */ > > > > > __virtio64 addr; > > > > > /* Length. */ > > > > > __virtio32 len; > > > > > /* The flags as indicated above. */ > > > > > __virtio16 flags; > > > > > /* We chain unused descriptors via this, too */ > > > > > __virtio16 next; > > > > >=20 > > > > > }; > > > >=20 > > > > Yes, Split Virtqueues have a fundamental limit on indirect table si= ze > > > > due to the "next" field. Packed Virtqueue descriptors don't have a > > > > "next" field so descriptor chains could be longer in theory (curren= tly > > > > forbidden by the spec). > > > >=20 > > > > > > One thing that's messy is that we've been discussing the maximu= m > > > > > > descriptor chain length but 9p has the "msize" concept, which i= sn't > > > > > > aware of contiguous memory. It may be necessary to extend the 9= p > > > > > > driver > > > > > > code to size requests not just according to their length in byt= es > > > > > > but > > > > > > also according to the descriptor chain length. That's how the L= inux > > > > > > block layer deals with queue limits (struct queue_limits > > > > > > max_segments vs > > > > > > max_hw_sectors). > > > > >=20 > > > > > Hmm, can't follow on that one. For what should that be needed in = case > > > > > of > > > > > 9p? My plan was to limit msize by 9p client simply at session sta= rt to > > > > > whatever is the max. amount virtio descriptors supported by host = and > > > > > using PAGE_SIZE as size per descriptor, because that's what 9p cl= ient > > > > > actually does ATM (see above). So you think that should be change= d to > > > > > e.g. just one descriptor for 4MB, right? > > > >=20 > > > > Limiting msize to the 9p transport device's maximum number of > > > > descriptors is conservative (i.e. 128 descriptors =3D 512 KB msize) > > > > because it doesn't take advantage of contiguous memory. I suggest > > > > leaving msize alone, adding a separate limit at which requests are = split > > > > according to the maximum descriptor chain length, and tweaking > > > > pack_sg_list() to coalesce adjacent pages. > > > >=20 > > > > That way msize can be large without necessarily using lots of > > > > descriptors (depending on the memory layout). > > >=20 > > > That was actually a tempting solution. Because it would neither requi= re > > > changes to the virtio specs (at least for a while) and it would also = work > > > with older QEMU versions. And for that pack_sg_list() portion of the = code > > > it would work well and easy as the buffer passed to pack_sg_list() is > > > contiguous already. > > >=20 > > > However I just realized for the zero-copy version of the code that wo= uld > > > be > > > more tricky. The ZC version already uses individual pages (struct pag= e, > > > hence PAGE_SIZE each) which are pinned, i.e. it uses pack_sg_list_p()= [1] > > > in combination with p9_get_mapped_pages() [2] > > >=20 > > > [1] > > > https://github.com/torvalds/linux/blob/7ddb58cb0ecae8e8b6181d736a8766= 7cc9 > > > ab8389/net/9p/trans_virtio.c#L218 [2] > > > https://github.com/torvalds/linux/blob/7ddb58cb0ecae8e8b6181d736a8766= 7cc9 > > > ab8389/net/9p/trans_virtio.c#L309 > > >=20 > > > So that would require much more work and code trying to sort and coal= esce > > > individual pages to contiguous physical memory for the sake of reduci= ng > > > virtio descriptors. And there is no guarantee that this is even possi= ble. > > > The kernel may simply return a non-contiguous set of pages which woul= d > > > eventually end up exceeding the virtio descriptor limit again. > >=20 > > Order must be preserved so pages cannot be sorted by physical address. > > How about simply coalescing when pages are adjacent? >=20 > It would help, but not solve the issue we are talking about here: if 99% = of=20 > the cases could successfully merge descriptors to stay below the descript= or=20 > count limit, but in 1% of the cases it could not, then this still constru= es a=20 > severe runtime issue that could trigger at any time. >=20 > > > So looks like it was probably still easier and realistic to just add > > > virtio > > > capabilities for now for allowing to exceed current descriptor limit. > >=20 > > I'm still not sure why virtio-net, virtio-blk, virtio-fs, etc perform > > fine under today's limits while virtio-9p needs a much higher limit to > > achieve good performance. Maybe there is an issue in a layer above the > > vring that's causing the virtio-9p performance you've observed? >=20 > Are you referring to (somewhat) recent benchmarks when saying those would= all=20 > still perform fine today? I'm not referring to specific benchmark results. Just that none of those devices needed to raise the descriptor chain length, so I'm surprised that virtio-9p needs it because it's conceptually similar to these devices. > Vivek was running detailed benchmarks for virtiofs vs. 9p: > https://lists.gnu.org/archive/html/qemu-devel/2020-12/msg02704.html >=20 > For the virtio aspect discussed here, only the benchmark configurations= =20 > without cache are relevant (9p-none, vtfs-none) and under this aspect the= =20 > situation seems to be quite similar between 9p and virtio-fs. You'll also= note=20 > once DAX is enabled (vtfs-none-dax) that apparently boosts virtio-fs=20 > performance significantly, which however seems to corelate to numbers whe= n I=20 > am running 9p with msize > 300k. Note: Vivek was presumably running 9p=20 > effecively with msize=3D300k, as this was the kernel limitation at that t= ime. Agreed, virtio-9p and virtiofs are similar without caching. I think we shouldn't consider DAX here since it bypasses the virtqueue. > To bring things into relation: there are known performance aspects in 9p = that=20 > can be improved, yes, both on Linux kernel side and on 9p server side in = QEMU.=20 > For instance 9p server uses coroutines [1] and currently dispatches betwe= en=20 > worker thread(s) and main thread too often per request (partly addressed= =20 > already [2], but still WIP), which accumulates to overall latency. But Vi= vek=20 > was actually using a 9p patch here which disabled coroutines entirely, wh= ich=20 > suggests that the virtio transmission size limit still represents a=20 > bottleneck. These results were collected with 4k block size. Neither msize nor the descriptor chain length limits will be stressed, so I don't think these results are relevant here. Maybe a more relevant comparison would be virtio-9p, virtiofs, and virtio-blk when block size is large (e.g. 1M). The Linux block layer in the guest will split virtio-blk requests when they exceed the block queue limits. Stefan >=20 > [1] https://wiki.qemu.org/Documentation/9p#Coroutines > [2] https://wiki.qemu.org/Documentation/9p#Implementation_Plans >=20 > Best regards, > Christian Schoenebeck >=20 >=20 --eestfZp/R6WSRbB0 Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEhpWov9P5fNqsNXdanKSrs4Grc8gFAmGLmX4ACgkQnKSrs4Gr c8ib+Qf/SKVx+e+woQwfvKZrZAOngttbg1kfU9RmuRPfwP2+OWFJFTfGlZAQFDr0 fwZRAZV15JXseHW1CkEwe5SUq7doi6PaVdmL5FaNHxmKJ+XwdMRmq/TVMA2KA6w2 Ny+hxinVZzRN1MDknaB6rB6qv3w2X/kkxZvKF6s7iNDYEYzITfri29w7lK6NI+6V 0xOg7ivlUqKYTJ3JIHYq73F/imKmVUR34AFGDFKAFlmSuOiGjvqltmSm3GEwbWzb fZFp+nf0QqPG+FBtu9ixhZ7Svq2yjMy6eYYyrSRpIpOZWJNF/kP0ziSylaq2/oHp FoA/coxSaKhMvlM4s2/upEMtxQRT2Q== =ytbs -----END PGP SIGNATURE----- --eestfZp/R6WSRbB0-- From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Wed, 10 Nov 2021 10:05:50 +0000 From: Stefan Hajnoczi Message-ID: References: <10421947.PG2v2feK4y@silver> <25571471.tMsSMU6axZ@silver> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="eestfZp/R6WSRbB0" Content-Disposition: inline In-Reply-To: <25571471.tMsSMU6axZ@silver> Subject: Re: [Virtio-fs] [PATCH v2 0/3] virtio: increase VIRTQUEUE_MAX_SIZE to 32k List-Id: Development discussions about virtio-fs List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Christian Schoenebeck Cc: Kevin Wolf , Laurent Vivier , qemu-block@nongnu.org, "Michael S. Tsirkin" , Jason Wang , Amit Shah , David Hildenbrand , qemu-devel@nongnu.org, virtio-fs@redhat.com, Eric Auger , Hanna Reitz , "Gonglei (Arei)" , Gerd Hoffmann , =?iso-8859-1?Q?Marc-Andr=E9?= Lureau , Paolo Bonzini , Fam Zheng , Raphael Norwitz --eestfZp/R6WSRbB0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Nov 09, 2021 at 02:09:59PM +0100, Christian Schoenebeck wrote: > On Dienstag, 9. November 2021 11:56:35 CET Stefan Hajnoczi wrote: > > On Thu, Nov 04, 2021 at 03:41:23PM +0100, Christian Schoenebeck wrote: > > > On Mittwoch, 3. November 2021 12:33:33 CET Stefan Hajnoczi wrote: > > > > On Mon, Nov 01, 2021 at 09:29:26PM +0100, Christian Schoenebeck wro= te: > > > > > On Donnerstag, 28. Oktober 2021 11:00:48 CET Stefan Hajnoczi wrot= e: > > > > > > On Mon, Oct 25, 2021 at 05:03:25PM +0200, Christian Schoenebeck= =20 > wrote: > > > > > > > On Montag, 25. Oktober 2021 12:30:41 CEST Stefan Hajnoczi wro= te: > > > > > > > > On Thu, Oct 21, 2021 at 05:39:28PM +0200, Christian Schoene= beck=20 > wrote: > > > > > > > > > On Freitag, 8. Oktober 2021 18:08:48 CEST Christian=20 > Schoenebeck wrote: > > > > > > > > > > On Freitag, 8. Oktober 2021 16:24:42 CEST Christian=20 > Schoenebeck wrote: > > > > > > > > > > > On Freitag, 8. Oktober 2021 09:25:33 CEST Greg Kurz w= rote: > > > > > > > > > > > > On Thu, 7 Oct 2021 16:42:49 +0100 > > > > > > > > > > > >=20 > > > > > > > > > > > > Stefan Hajnoczi wrote: > > > > > > > > > > > > > On Thu, Oct 07, 2021 at 02:51:55PM +0200, Christi= an=20 > Schoenebeck wrote: > > > > > > > > > > > > > > On Donnerstag, 7. Oktober 2021 07:23:59 CEST St= efan=20 > Hajnoczi wrote: > > > > > > > > > > > > > > > On Mon, Oct 04, 2021 at 09:38:00PM +0200, > > > > > > > > > > > > > > > Christian > > > > > > > > > > > > > > > Schoenebeck > > > > > > > > > >=20 > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > At the moment the maximum transfer size with > > > > > > > > > > > > > > > > virtio > > > > > > > > > > > > > > > > is > > > > > > > > > > > > > > > > limited > > > > > > > > > > > > > > > > to > > > > > > > > > > > > > > > > 4M > > > > > > > > > > > > > > > > (1024 * PAGE_SIZE). This series raises this > > > > > > > > > > > > > > > > limit to > > > > > > > > > > > > > > > > its > > > > > > > > > > > > > > > > maximum > > > > > > > > > > > > > > > > theoretical possible transfer size of 128M = (32k > > > > > > > > > > > > > > > > pages) > > > > > > > > > > > > > > > > according > > > > > > > > > > > > > > > > to > > > > > > > > > > > > > > > > the > > > > > > > > > > > > > > > > virtio specs: > > > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > > > https://docs.oasis-open.org/virtio/virtio/v= 1.1/c > > > > > > > > > > > > > > > > s01/ > > > > > > > > > > > > > > > > virt > > > > > > > > > > > > > > > > io-v > > > > > > > > > > > > > > > > 1.1- > > > > > > > > > > > > > > > > cs > > > > > > > > > > > > > > > > 01 > > > > > > > > > > > > > > > > .html# > > > > > > > > > > > > > > > > x1-240006 > > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > > Hi Christian, > > > > > > > > > > > >=20 > > > > > > > > > > > > > > > I took a quick look at the code: > > > > > > > > > > > > Hi, > > > > > > > > > > > >=20 > > > > > > > > > > > > Thanks Stefan for sharing virtio expertise and help= ing > > > > > > > > > > > > Christian > > > > > > > > > > > > ! > > > > > > > > > > > >=20 > > > > > > > > > > > > > > > - The Linux 9p driver restricts descriptor ch= ains > > > > > > > > > > > > > > > to > > > > > > > > > > > > > > > 128 > > > > > > > > > > > > > > > elements > > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > > (net/9p/trans_virtio.c:VIRTQUEUE_NUM) > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > Yes, that's the limitation that I am about to r= emove > > > > > > > > > > > > > > (WIP); > > > > > > > > > > > > > > current > > > > > > > > > > > > > > kernel > > > > > > > > > > > > > > patches: > > > > > > > > > > > > > > https://lore.kernel.org/netdev/cover.1632327421= =2Egit. > > > > > > > > > > > > > > linu > > > > > > > > > > > > > > x_os > > > > > > > > > > > > > > s@cr > > > > > > > > > > > > > > udeb > > > > > > > > > > > > > > yt > > > > > > > > > > > > > > e. > > > > > > > > > > > > > > com/> > > > > > > > > > > > > >=20 > > > > > > > > > > > > > I haven't read the patches yet but I'm concerned = that > > > > > > > > > > > > > today > > > > > > > > > > > > > the > > > > > > > > > > > > > driver > > > > > > > > > > > > > is pretty well-behaved and this new patch series > > > > > > > > > > > > > introduces a > > > > > > > > > > > > > spec > > > > > > > > > > > > > violation. Not fixing existing spec violations is > > > > > > > > > > > > > okay, > > > > > > > > > > > > > but > > > > > > > > > > > > > adding > > > > > > > > > > > > > new > > > > > > > > > > > > > ones is a red flag. I think we need to figure out= a > > > > > > > > > > > > > clean > > > > > > > > > > > > > solution. > > > > > > > > > > >=20 > > > > > > > > > > > Nobody has reviewed the kernel patches yet. My main > > > > > > > > > > > concern > > > > > > > > > > > therefore > > > > > > > > > > > actually is that the kernel patches are already too > > > > > > > > > > > complex, > > > > > > > > > > > because > > > > > > > > > > > the > > > > > > > > > > > current situation is that only Dominique is handling = 9p > > > > > > > > > > > patches on > > > > > > > > > > > kernel > > > > > > > > > > > side, and he barely has time for 9p anymore. > > > > > > > > > > >=20 > > > > > > > > > > > Another reason for me to catch up on reading current > > > > > > > > > > > kernel > > > > > > > > > > > code > > > > > > > > > > > and > > > > > > > > > > > stepping in as reviewer of 9p on kernel side ASAP, > > > > > > > > > > > independent > > > > > > > > > > > of > > > > > > > > > > > this > > > > > > > > > > > issue. > > > > > > > > > > >=20 > > > > > > > > > > > As for current kernel patches' complexity: I can cert= ainly > > > > > > > > > > > drop > > > > > > > > > > > patch > > > > > > > > > > > 7 > > > > > > > > > > > entirely as it is probably just overkill. Patch 4 is = then > > > > > > > > > > > the > > > > > > > > > > > biggest > > > > > > > > > > > chunk, I have to see if I can simplify it, and whethe= r it > > > > > > > > > > > would > > > > > > > > > > > make > > > > > > > > > > > sense to squash with patch 3. > > > > > > > > > > >=20 > > > > > > > > > > > > > > > - The QEMU 9pfs code passes iovecs directly to > > > > > > > > > > > > > > > preadv(2) > > > > > > > > > > > > > > > and > > > > > > > > > > > > > > > will > > > > > > > > > > > > > > > fail > > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > > with EINVAL when called with more than IOV_= MAX > > > > > > > > > > > > > > > iovecs > > > > > > > > > > > > > > > (hw/9pfs/9p.c:v9fs_read()) > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > Hmm, which makes me wonder why I never encounte= red > > > > > > > > > > > > > > this > > > > > > > > > > > > > > error > > > > > > > > > > > > > > during > > > > > > > > > > > > > > testing. > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > Most people will use the 9p qemu 'local' fs dri= ver > > > > > > > > > > > > > > backend > > > > > > > > > > > > > > in > > > > > > > > > > > > > > practice, > > > > > > > > > > > > > > so > > > > > > > > > > > > > > that v9fs_read() call would translate for most > > > > > > > > > > > > > > people to > > > > > > > > > > > > > > this > > > > > > > > > > > > > > implementation on QEMU side (hw/9p/9p-local.c): > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > static ssize_t local_preadv(FsContext *ctx, > > > > > > > > > > > > > > V9fsFidOpenState > > > > > > > > > > > > > > *fs, > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > const struct iovec = *iov, > > > > > > > > > > > > > > int iovcnt, off_t > > > > > > > > > > > > > > offset) > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > { > > > > > > > > > > > > > > #ifdef CONFIG_PREADV > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > return preadv(fs->fd, iov, iovcnt, offset); > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > #else > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > int err =3D lseek(fs->fd, offset, SEEK_SET); > > > > > > > > > > > > > > if (err =3D=3D -1) { > > > > > > > > > > > > > > =20 > > > > > > > > > > > > > > return err; > > > > > > > > > > > > > > =20 > > > > > > > > > > > > > > } else { > > > > > > > > > > > > > > =20 > > > > > > > > > > > > > > return readv(fs->fd, iov, iovcnt); > > > > > > > > > > > > > > =20 > > > > > > > > > > > > > > } > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > #endif > > > > > > > > > > > > > > } > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > > Unless I misunderstood the code, neither side= can > > > > > > > > > > > > > > > take > > > > > > > > > > > > > > > advantage > > > > > > > > > > > > > > > of > > > > > > > > > > > > > > > the > > > > > > > > > > > > > > > new 32k descriptor chain limit? > > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > > > > > > Stefan > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > I need to check that when I have some more time= =2E One > > > > > > > > > > > > > > possible > > > > > > > > > > > > > > explanation > > > > > > > > > > > > > > might be that preadv() already has this wrapped= into > > > > > > > > > > > > > > a > > > > > > > > > > > > > > loop > > > > > > > > > > > > > > in > > > > > > > > > > > > > > its > > > > > > > > > > > > > > implementation to circumvent a limit like IOV_M= AX. > > > > > > > > > > > > > > It > > > > > > > > > > > > > > might > > > > > > > > > > > > > > be > > > > > > > > > > > > > > another > > > > > > > > > > > > > > "it > > > > > > > > > > > > > > works, but not portable" issue, but not sure. > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > There are still a bunch of other issues I have = to > > > > > > > > > > > > > > resolve. > > > > > > > > > > > > > > If > > > > > > > > > > > > > > you > > > > > > > > > > > > > > look > > > > > > > > > > > > > > at > > > > > > > > > > > > > > net/9p/client.c on kernel side, you'll notice t= hat > > > > > > > > > > > > > > it > > > > > > > > > > > > > > basically > > > > > > > > > > > > > > does > > > > > > > > > > > > > > this ATM> > > > > > > > > > > > > > > >=20 > > > > > > > > > > > > > > kmalloc(msize); > > > > > > > > > > > >=20 > > > > > > > > > > > > Note that this is done twice : once for the T messa= ge > > > > > > > > > > > > (client > > > > > > > > > > > > request) > > > > > > > > > > > > and > > > > > > > > > > > > once for the R message (server answer). The 9p driv= er > > > > > > > > > > > > could > > > > > > > > > > > > adjust > > > > > > > > > > > > the > > > > > > > > > > > > size > > > > > > > > > > > > of the T message to what's really needed instead of > > > > > > > > > > > > allocating > > > > > > > > > > > > the > > > > > > > > > > > > full > > > > > > > > > > > > msize. R message size is not known though. > > > > > > > > > > >=20 > > > > > > > > > > > Would it make sense adding a second virtio ring, dedi= cated > > > > > > > > > > > to > > > > > > > > > > > server > > > > > > > > > > > responses to solve this? IIRC 9p server already calcu= lates > > > > > > > > > > > appropriate > > > > > > > > > > > exact sizes for each response type. So server could j= ust > > > > > > > > > > > push > > > > > > > > > > > space > > > > > > > > > > > that's > > > > > > > > > > > really needed for its responses. > > > > > > > > > > >=20 > > > > > > > > > > > > > > for every 9p request. So not only does it alloc= ate > > > > > > > > > > > > > > much > > > > > > > > > > > > > > more > > > > > > > > > > > > > > memory > > > > > > > > > > > > > > for > > > > > > > > > > > > > > every request than actually required (i.e. say = 9pfs > > > > > > > > > > > > > > was > > > > > > > > > > > > > > mounted > > > > > > > > > > > > > > with > > > > > > > > > > > > > > msize=3D8M, then a 9p request that actually wou= ld just > > > > > > > > > > > > > > need 1k > > > > > > > > > > > > > > would > > > > > > > > > > > > > > nevertheless allocate 8M), but also it allocate= s > > > > > > > > > > > > > > > PAGE_SIZE, > > > > > > > > > > > > > > which > > > > > > > > > > > > > > obviously may fail at any time.> > > > > > > > > > > > > >=20 > > > > > > > > > > > > > The PAGE_SIZE limitation sounds like a kmalloc() = vs > > > > > > > > > > > > > vmalloc() > > > > > > > > > > > > > situation. > > > > > > > > > > >=20 > > > > > > > > > > > Hu, I didn't even consider vmalloc(). I just tried the > > > > > > > > > > > kvmalloc() > > > > > > > > > > > wrapper > > > > > > > > > > > as a quick & dirty test, but it crashed in the same w= ay as > > > > > > > > > > > kmalloc() > > > > > > > > > > > with > > > > > > > > > > > large msize values immediately on mounting: > > > > > > > > > > >=20 > > > > > > > > > > > diff --git a/net/9p/client.c b/net/9p/client.c > > > > > > > > > > > index a75034fa249b..cfe300a4b6ca 100644 > > > > > > > > > > > --- a/net/9p/client.c > > > > > > > > > > > +++ b/net/9p/client.c > > > > > > > > > > > @@ -227,15 +227,18 @@ static int parse_opts(char *opt= s, > > > > > > > > > > > struct > > > > > > > > > > > p9_client > > > > > > > > > > > *clnt) > > > > > > > > > > >=20 > > > > > > > > > > > static int p9_fcall_init(struct p9_client *c, struct > > > > > > > > > > > p9_fcall > > > > > > > > > > > *fc, > > > > > > > > > > > =20 > > > > > > > > > > > int alloc_msize) > > > > > > > > > > > =20 > > > > > > > > > > > { > > > > > > > > > > >=20 > > > > > > > > > > > - if (likely(c->fcall_cache) && alloc_msize =3D= =3D > > > > > > > > > > > c->msize) > > > > > > > > > > > { > > > > > > > > > > > + //if (likely(c->fcall_cache) && alloc_msize = =3D=3D > > > > > > > > > > > c->msize) { > > > > > > > > > > > + if (false) { > > > > > > > > > > >=20 > > > > > > > > > > > fc->sdata =3D > > > > > > > > > > > kmem_cache_alloc(c->fcall_cache, > > > > > > > > > > > GFP_NOFS); > > > > > > > > > > > fc->cache =3D c->fcall_cache; > > > > > > > > > > > =20 > > > > > > > > > > > } else { > > > > > > > > > > >=20 > > > > > > > > > > > - fc->sdata =3D kmalloc(alloc_msize, > > > > > > > > > > > GFP_NOFS); > > > > > > > > > > > + fc->sdata =3D kvmalloc(alloc_msize, > > > > > > > > > > > GFP_NOFS); > > > > > > > > > >=20 > > > > > > > > > > Ok, GFP_NOFS -> GFP_KERNEL did the trick. > > > > > > > > > >=20 > > > > > > > > > > Now I get: > > > > > > > > > > virtio: bogus descriptor or out of resources > > > > > > > > > >=20 > > > > > > > > > > So, still some work ahead on both ends. > > > > > > > > >=20 > > > > > > > > > Few hacks later (only changes on 9p client side) I got th= is > > > > > > > > > running > > > > > > > > > stable > > > > > > > > > now. The reason for the virtio error above was that kvmal= loc() > > > > > > > > > returns > > > > > > > > > a > > > > > > > > > non-logical kernel address for any kvmalloc(>4M), i.e. an > > > > > > > > > address > > > > > > > > > that > > > > > > > > > is > > > > > > > > > inaccessible from host side, hence that "bogus descriptor" > > > > > > > > > message > > > > > > > > > by > > > > > > > > > QEMU. > > > > > > > > > So I had to split those linear 9p client buffers into spa= rse > > > > > > > > > ones > > > > > > > > > (set > > > > > > > > > of > > > > > > > > > individual pages). > > > > > > > > >=20 > > > > > > > > > I tested this for some days with various virtio transmiss= ion > > > > > > > > > sizes > > > > > > > > > and > > > > > > > > > it > > > > > > > > > works as expected up to 128 MB (more precisely: 128 MB re= ad > > > > > > > > > space > > > > > > > > > + > > > > > > > > > 128 MB > > > > > > > > > write space per virtio round trip message). > > > > > > > > >=20 > > > > > > > > > I did not encounter a show stopper for large virtio > > > > > > > > > transmission > > > > > > > > > sizes > > > > > > > > > (4 MB ... 128 MB) on virtio level, neither as a result of > > > > > > > > > testing, > > > > > > > > > nor > > > > > > > > > after reviewing the existing code. > > > > > > > > >=20 > > > > > > > > > About IOV_MAX: that's apparently not an issue on virtio l= evel. > > > > > > > > > Most of > > > > > > > > > the > > > > > > > > > iovec code, both on Linux kernel side and on QEMU side do= not > > > > > > > > > have > > > > > > > > > this > > > > > > > > > limitation. It is apparently however indeed a limitation = for > > > > > > > > > userland > > > > > > > > > apps > > > > > > > > > calling the Linux kernel's syscalls yet. > > > > > > > > >=20 > > > > > > > > > Stefan, as it stands now, I am even more convinced that t= he > > > > > > > > > upper > > > > > > > > > virtio > > > > > > > > > transmission size limit should not be squeezed into the q= ueue > > > > > > > > > size > > > > > > > > > argument of virtio_add_queue(). Not because of the previo= us > > > > > > > > > argument > > > > > > > > > that > > > > > > > > > it would waste space (~1MB), but rather because they are = two > > > > > > > > > different > > > > > > > > > things. To outline this, just a quick recap of what happe= ns > > > > > > > > > exactly > > > > > > > > > when > > > > > > > > > a bulk message is pushed over the virtio wire (assuming v= irtio > > > > > > > > > "split" > > > > > > > > > layout here): > > > > > > > > >=20 > > > > > > > > > ---------- [recap-start] ---------- > > > > > > > > >=20 > > > > > > > > > For each bulk message sent guest <-> host, exactly *one* = of > > > > > > > > > the > > > > > > > > > pre-allocated descriptors is taken and placed (subsequent= ly) > > > > > > > > > into > > > > > > > > > exactly > > > > > > > > > *one* position of the two available/used ring buffers. The > > > > > > > > > actual > > > > > > > > > descriptor table though, containing all the DMA addresses= of > > > > > > > > > the > > > > > > > > > message > > > > > > > > > bulk data, is allocated just in time for each round trip > > > > > > > > > message. > > > > > > > > > Say, > > > > > > > > > it > > > > > > > > > is the first message sent, it yields in the following > > > > > > > > > structure: > > > > > > > > >=20 > > > > > > > > > Ring Buffer Descriptor Table Bulk Data Pages > > > > > > > > >=20 > > > > > > > > > +-+ +-+ +-----------------+ > > > > > > > > > =20 > > > > > > > > > |D|------------->|d|---------->| Bulk data block | > > > > > > > > > =20 > > > > > > > > > +-+ |d|--------+ +-----------------+ > > > > > > > > > =20 > > > > > > > > > | | |d|------+ | > > > > > > > > > =20 > > > > > > > > > +-+ . | | +-----------------+ > > > > > > > > > =20 > > > > > > > > > | | . | +->| Bulk data block | > > > > > > > > > =20 > > > > > > > > > . . | +-----------------+ > > > > > > > > > . |d|-+ | > > > > > > > > > . +-+ | | +-----------------+ > > > > > > > > > =20 > > > > > > > > > | | | +--->| Bulk data block | > > > > > > > > > =20 > > > > > > > > > +-+ | +-----------------+ > > > > > > > > > =20 > > > > > > > > > | | | . > > > > > > > > > =20 > > > > > > > > > +-+ | . > > > > > > > > > =20 > > > > > > > > > | . > > > > > > > > > | =20 > > > > > > > > > | +-----------------+ > > > > > > > > > =20 > > > > > > > > > +-------->| Bulk data block | > > > > > > > > > =20 > > > > > > > > > +-----------------+ > > > > > > > > >=20 > > > > > > > > > Legend: > > > > > > > > > D: pre-allocated descriptor > > > > > > > > > d: just in time allocated descriptor > > > > > > > > > -->: memory pointer (DMA) > > > > > > > > >=20 > > > > > > > > > The bulk data blocks are allocated by the respective devi= ce > > > > > > > > > driver > > > > > > > > > above > > > > > > > > > virtio subsystem level (guest side). > > > > > > > > >=20 > > > > > > > > > There are exactly as many descriptors pre-allocated (D) a= s the > > > > > > > > > size of > > > > > > > > > a > > > > > > > > > ring buffer. > > > > > > > > >=20 > > > > > > > > > A "descriptor" is more or less just a chainable DMA memory > > > > > > > > > pointer; > > > > > > > > > defined > > > > > > > > > as: > > > > > > > > >=20 > > > > > > > > > /* Virtio ring descriptors: 16 bytes. These can chain > > > > > > > > > together > > > > > > > > > via > > > > > > > > > "next". */ struct vring_desc { > > > > > > > > >=20 > > > > > > > > > /* Address (guest-physical). */ > > > > > > > > > __virtio64 addr; > > > > > > > > > /* Length. */ > > > > > > > > > __virtio32 len; > > > > > > > > > /* The flags as indicated above. */ > > > > > > > > > __virtio16 flags; > > > > > > > > > /* We chain unused descriptors via this, too */ > > > > > > > > > __virtio16 next; > > > > > > > > >=20 > > > > > > > > > }; > > > > > > > > >=20 > > > > > > > > > There are 2 ring buffers; the "available" ring buffer is = for > > > > > > > > > sending a > > > > > > > > > message guest->host (which will transmit DMA addresses of > > > > > > > > > guest > > > > > > > > > allocated > > > > > > > > > bulk data blocks that are used for data sent to device, a= nd > > > > > > > > > separate > > > > > > > > > guest allocated bulk data blocks that will be used by host > > > > > > > > > side to > > > > > > > > > place > > > > > > > > > its response bulk data), and the "used" ring buffer is for > > > > > > > > > sending > > > > > > > > > host->guest to let guest know about host's response and t= hat > > > > > > > > > it > > > > > > > > > could > > > > > > > > > now > > > > > > > > > safely consume and then deallocate the bulk data blocks > > > > > > > > > subsequently. > > > > > > > > >=20 > > > > > > > > > ---------- [recap-end] ---------- > > > > > > > > >=20 > > > > > > > > > So the "queue size" actually defines the ringbuffer size.= It > > > > > > > > > does > > > > > > > > > not > > > > > > > > > define the maximum amount of descriptors. The "queue size" > > > > > > > > > rather > > > > > > > > > defines > > > > > > > > > how many pending messages can be pushed into either one > > > > > > > > > ringbuffer > > > > > > > > > before > > > > > > > > > the other side would need to wait until the counter side = would > > > > > > > > > step up > > > > > > > > > (i.e. ring buffer full). > > > > > > > > >=20 > > > > > > > > > The maximum amount of descriptors (what VIRTQUEUE_MAX_SIZE > > > > > > > > > actually > > > > > > > > > is) > > > > > > > > > OTOH defines the max. bulk data size that could be transm= itted > > > > > > > > > with > > > > > > > > > each > > > > > > > > > virtio round trip message. > > > > > > > > >=20 > > > > > > > > > And in fact, 9p currently handles the virtio "queue size"= as > > > > > > > > > directly > > > > > > > > > associative with its maximum amount of active 9p requests= the > > > > > > > > > server > > > > > > > > > could > > > > > > > > >=20 > > > > > > > > > handle simultaniously: > > > > > > > > > hw/9pfs/9p.h:#define MAX_REQ 128 > > > > > > > > > hw/9pfs/9p.h: V9fsPDU pdus[MAX_REQ]; > > > > > > > > > hw/9pfs/virtio-9p-device.c: v->vq =3D > > > > > > > > > virtio_add_queue(vdev, > > > > > > > > > MAX_REQ, > > > > > > > > > =20 > > > > > > > > > handle_9p_output); > > > > > > > > >=20 > > > > > > > > > So if I would change it like this, just for the purpose to > > > > > > > > > increase > > > > > > > > > the > > > > > > > > > max. virtio transmission size: > > > > > > > > >=20 > > > > > > > > > --- a/hw/9pfs/virtio-9p-device.c > > > > > > > > > +++ b/hw/9pfs/virtio-9p-device.c > > > > > > > > > @@ -218,7 +218,7 @@ static void > > > > > > > > > virtio_9p_device_realize(DeviceState > > > > > > > > > *dev, > > > > > > > > > Error **errp)> > > > > > > > > >=20 > > > > > > > > > v->config_size =3D sizeof(struct virtio_9p_config) + > > > > > > > > > strlen(s->fsconf.tag); > > > > > > > > > virtio_init(vdev, "virtio-9p", VIRTIO_ID_9P, > > > > > > > > > v->config_size, > > > > > > > > > =20 > > > > > > > > > VIRTQUEUE_MAX_SIZE); > > > > > > > > >=20 > > > > > > > > > - v->vq =3D virtio_add_queue(vdev, MAX_REQ, > > > > > > > > > handle_9p_output); > > > > > > > > > + v->vq =3D virtio_add_queue(vdev, 32*1024, > > > > > > > > > handle_9p_output); > > > > > > > > >=20 > > > > > > > > > } > > > > > > > > >=20 > > > > > > > > > Then it would require additional synchronization code on = both > > > > > > > > > ends > > > > > > > > > and > > > > > > > > > therefore unnecessary complexity, because it would now be > > > > > > > > > possible > > > > > > > > > that > > > > > > > > > more requests are pushed into the ringbuffer than server = could > > > > > > > > > handle. > > > > > > > > >=20 > > > > > > > > > There is one potential issue though that probably did jus= tify > > > > > > > > > the > > > > > > > > > "don't > > > > > > > > > exceed the queue size" rule: > > > > > > > > >=20 > > > > > > > > > ATM the descriptor table is allocated (just in time) as *= one* > > > > > > > > > continuous > > > > > > > > > buffer via kmalloc_array(): > > > > > > > > > https://github.com/torvalds/linux/blob/2f111a6fd5b5297b4e= 92f53 > > > > > > > > > 798c > > > > > > > > > a086 > > > > > > > > > f7c7 > > > > > > > > > d33a4/drivers/virtio/virtio_ring.c#L440 > > > > > > > > >=20 > > > > > > > > > So assuming transmission size of 2 * 128 MB that > > > > > > > > > kmalloc_array() > > > > > > > > > call > > > > > > > > > would > > > > > > > > > yield in kmalloc(1M) and the latter might fail if guest h= ad > > > > > > > > > highly > > > > > > > > > fragmented physical memory. For such kind of error case t= here > > > > > > > > > is > > > > > > > > > currently a fallback path in virtqueue_add_split() that w= ould > > > > > > > > > then > > > > > > > > > use > > > > > > > > > the required amount of pre-allocated descriptors instead: > > > > > > > > > https://github.com/torvalds/linux/blob/2f111a6fd5b5297b4e= 92f53 > > > > > > > > > 798c > > > > > > > > > a086 > > > > > > > > > f7c7 > > > > > > > > > d33a4/drivers/virtio/virtio_ring.c#L525 > > > > > > > > >=20 > > > > > > > > > That fallback recovery path would no longer be viable if = the > > > > > > > > > queue > > > > > > > > > size > > > > > > > > > was > > > > > > > > > exceeded. There would be alternatives though, e.g. by all= owing > > > > > > > > > to > > > > > > > > > chain > > > > > > > > > indirect descriptor tables (currently prohibited by the v= irtio > > > > > > > > > specs). > > > > > > > >=20 > > > > > > > > Making the maximum number of descriptors independent of the > > > > > > > > queue > > > > > > > > size > > > > > > > > requires a change to the VIRTIO spec since the two values a= re > > > > > > > > currently > > > > > > > > explicitly tied together by the spec. > > > > > > >=20 > > > > > > > Yes, that's what the virtio specs say. But they don't say why= , nor > > > > > > > did > > > > > > > I > > > > > > > hear a reason in this dicussion. > > > > > > >=20 > > > > > > > That's why I invested time reviewing current virtio implement= ation > > > > > > > and > > > > > > > specs, as well as actually testing exceeding that limit. And = as I > > > > > > > outlined in detail in my previous email, I only found one > > > > > > > theoretical > > > > > > > issue that could be addressed though. > > > > > >=20 > > > > > > I agree that there is a limitation in the VIRTIO spec, but viol= ating > > > > > > the > > > > > > spec isn't an acceptable solution: > > > > > >=20 > > > > > > 1. QEMU and Linux aren't the only components that implement VIR= TIO. > > > > > > You > > > > > >=20 > > > > > > cannot make assumptions about their implementations because = it > > > > > > may > > > > > > break spec-compliant implementations that you haven't looked= at. > > > > > > =20 > > > > > > Your patches weren't able to increase Queue Size because some > > > > > > device > > > > > > implementations break when descriptor chains are too long. T= his > > > > > > shows > > > > > > there is a practical issue even in QEMU. > > > > > >=20 > > > > > > 2. The specific spec violation that we discussed creates the pr= oblem > > > > > >=20 > > > > > > that drivers can no longer determine the maximum description > > > > > > chain > > > > > > length. This in turn will lead to more implementation-specif= ic > > > > > > assumptions being baked into drivers and cause problems with > > > > > > interoperability and future changes. > > > > > >=20 > > > > > > The spec needs to be extended instead. I included an idea for h= ow to > > > > > > do > > > > > > that below. > > > > >=20 > > > > > Sure, I just wanted to see if there was a non-neglectable "hard" = show > > > > > stopper per se that I probably haven't seen yet. I have not quest= ioned > > > > > aiming a clean solution. > > > > >=20 > > > > > Thanks for the clarification! > > > > >=20 > > > > > > > > Before doing that, are there benchmark results showing that= 1 MB > > > > > > > > vs > > > > > > > > 128 > > > > > > > > MB produces a performance improvement? I'm asking because if > > > > > > > > performance > > > > > > > > with 1 MB is good then you can probably do that without hav= ing > > > > > > > > to > > > > > > > > change > > > > > > > > VIRTIO and also because it's counter-intuitive that 9p need= s 128 > > > > > > > > MB > > > > > > > > for > > > > > > > > good performance when it's ultimately implemented on top of= disk > > > > > > > > and > > > > > > > > network I/O that have lower size limits. > > > > > > >=20 > > > > > > > First some numbers, linear reading a 12 GB file: > > > > > > >=20 > > > > > > > msize average notes > > > > > > >=20 > > > > > > > 8 kB 52.0 MB/s default msize of Linux kernel > > > > > > 128 kB 624.8 MB/s default msize of Linux kernel >=3Dv5.15 > > > > > > > 512 kB 1961 MB/s current max. msize with any Linux kernel > > > > > > > <=3Dv5.15 > > > > > > > 1 MB 2551 MB/s this msize would already violate virtio > > > > > > > specs > > > > > > > 2 MB 2521 MB/s this msize would already violate virtio > > > > > > > specs > > > > > > > 4 MB 2628 MB/s planned max. msize of my current kernel > > > > > > > patches > > > > > > > [1] > > > > > >=20 > > > > > > How many descriptors are used? 4 MB can be covered by a single > > > > > > descriptor if the data is physically contiguous in memory, so t= his > > > > > > data > > > > > > doesn't demonstrate a need for more descriptors. > > > > >=20 > > > > > No, in the last couple years there was apparently no kernel versi= on > > > > > that > > > > > used just one descriptor, nor did my benchmarked version. Even th= ough > > > > > the > > > > > Linux 9p client uses (yet) simple linear buffers (contiguous phys= ical > > > > > memory) on 9p client level, these are however split into PAGE_SIZE > > > > > chunks > > > > > by function pack_sg_list() [1] before being fed to virtio level: > > > > >=20 > > > > > static unsigned int rest_of_page(void *data) > > > > > { > > > > >=20 > > > > > return PAGE_SIZE - offset_in_page(data); > > > > >=20 > > > > > } > > > > > ... > > > > > static int pack_sg_list(struct scatterlist *sg, int start, > > > > >=20 > > > > > int limit, char *data, int count) > > > > >=20 > > > > > { > > > > >=20 > > > > > int s; > > > > > int index =3D start; > > > > > =09 > > > > > while (count) { > > > > > =09 > > > > > s =3D rest_of_page(data); > > > > > ... > > > > > sg_set_buf(&sg[index++], data, s); > > > > > count -=3D s; > > > > > data +=3D s; > > > > > =09 > > > > > } > > > > > ... > > > > >=20 > > > > > } > > > > >=20 > > > > > [1] > > > > > https://github.com/torvalds/linux/blob/19901165d90fdca1e57c9baa0d= 5b4c6 > > > > > 3d1 > > > > > 5c476a/net/9p/trans_virtio.c#L171 > > > > >=20 > > > > > So when sending 4MB over virtio wire, it would yield in 1k descri= ptors > > > > > ATM. > > > > >=20 > > > > > I have wondered about this before, but did not question it, becau= se > > > > > due to > > > > > the cross-platform nature I couldn't say for certain whether that= 's > > > > > probably needed somewhere. I mean for the case virtio-PCI I know = for > > > > > sure > > > > > that one descriptor (i.e. >PAGE_SIZE) would be fine, but I don't = know > > > > > if > > > > > that applies to all buses and architectures. > > > >=20 > > > > VIRTIO does not limit descriptor the descriptor len field to PAGE_S= IZE, > > > > so I don't think there is a limit at the VIRTIO level. > > >=20 > > > So you are viewing this purely from virtio specs PoV: in the sense, i= f it > > > is not prohibited by the virtio specs, then it should work. Maybe. > >=20 > > Limitations must be specified either in the 9P protocol or the VIRTIO > > specification. Drivers and devices will not be able to operate correctly > > if there are limitations that aren't covered by the specs. > >=20 > > Do you have something in mind that isn't covered by the specs? >=20 > Not sure whether that's something that should be specified by the virtio= =20 > specs, probably not. I simply do not know if there was any bus or archite= cture=20 > that would have a limitation for max. size for a memory block passed per = one=20 > DMA address. Host-side limitations like that can exist. For example when a physical storage device on the host has limits that the VIRTIO device does not have. In this case both virtio-scsi and virtio-blk report those limits to the guest so that the guest won't submit requests that the physical device would reject. I guess networking MTU is kind of similar too. What they have in common is that the limit needs to be reported to the guest, typically using a VIRTIO Configuration Space field. It is an explicit limit that is part of the host<->guest interface (VIRTIO spec, SCSI, etc). > > > > If this function coalesces adjacent pages then the descriptor chain > > > > length issues could be reduced. > > > >=20 > > > > > > > But again, this is not just about performance. My conclusion = as > > > > > > > described > > > > > > > in my previous email is that virtio currently squeezes > > > > > > >=20 > > > > > > > "max. simultanious amount of bulk messages" > > > > > > >=20 > > > > > > > vs. > > > > > > >=20 > > > > > > > "max. bulk data transmission size per bulk messaage" > > > > > > >=20 > > > > > > > into the same configuration parameter, which is IMO inappropr= iate > > > > > > > and > > > > > > > hence > > > > > > > splitting them into 2 separate parameters when creating a que= ue > > > > > > > makes > > > > > > > sense, independent of the performance benchmarks. > > > > > > >=20 > > > > > > > [1] > > > > > > > https://lore.kernel.org/netdev/cover.1632327421.git.linux_oss= @crud > > > > > > > ebyt > > > > > > > e.c > > > > > > > om/ > > > > > >=20 > > > > > > Some devices effectively already have this because the device > > > > > > advertises > > > > > > a maximum number of descriptors via device-specific mechanisms = like > > > > > > the > > > > > > struct virtio_blk_config seg_max field. But today these fields = can > > > > > > only > > > > > > reduce the maximum descriptor chain length because the spec sti= ll > > > > > > limits > > > > > > the length to Queue Size. > > > > > >=20 > > > > > > We can build on this approach to raise the length above Queue S= ize. > > > > > > This > > > > > > approach has the advantage that the maximum number of segments = isn't > > > > > > per > > > > > > device or per virtqueue, it's fine-grained. If the device suppo= rts > > > > > > two > > > > > > requests types then different max descriptor chain limits could= be > > > > > > given > > > > > > for them by introducing two separate configuration space fields. > > > > > >=20 > > > > > > Here are the corresponding spec changes: > > > > > >=20 > > > > > > 1. A new feature bit called VIRTIO_RING_F_LARGE_INDIRECT_DESC is > > > > > > added > > > > > >=20 > > > > > > to indicate that indirect descriptor table size and maximum > > > > > > descriptor chain length are not limited by Queue Size value. > > > > > > (Maybe > > > > > > there still needs to be a limit like 2^15?) > > > > >=20 > > > > > Sounds good to me! > > > > >=20 > > > > > AFAIK it is effectively limited to 2^16 because of vring_desc->ne= xt: > > > > >=20 > > > > > /* Virtio ring descriptors: 16 bytes. These can chain together v= ia > > > > > "next". */ struct vring_desc { > > > > >=20 > > > > > /* Address (guest-physical). */ > > > > > __virtio64 addr; > > > > > /* Length. */ > > > > > __virtio32 len; > > > > > /* The flags as indicated above. */ > > > > > __virtio16 flags; > > > > > /* We chain unused descriptors via this, too */ > > > > > __virtio16 next; > > > > >=20 > > > > > }; > > > >=20 > > > > Yes, Split Virtqueues have a fundamental limit on indirect table si= ze > > > > due to the "next" field. Packed Virtqueue descriptors don't have a > > > > "next" field so descriptor chains could be longer in theory (curren= tly > > > > forbidden by the spec). > > > >=20 > > > > > > One thing that's messy is that we've been discussing the maximum > > > > > > descriptor chain length but 9p has the "msize" concept, which i= sn't > > > > > > aware of contiguous memory. It may be necessary to extend the 9p > > > > > > driver > > > > > > code to size requests not just according to their length in byt= es > > > > > > but > > > > > > also according to the descriptor chain length. That's how the L= inux > > > > > > block layer deals with queue limits (struct queue_limits > > > > > > max_segments vs > > > > > > max_hw_sectors). > > > > >=20 > > > > > Hmm, can't follow on that one. For what should that be needed in = case > > > > > of > > > > > 9p? My plan was to limit msize by 9p client simply at session sta= rt to > > > > > whatever is the max. amount virtio descriptors supported by host = and > > > > > using PAGE_SIZE as size per descriptor, because that's what 9p cl= ient > > > > > actually does ATM (see above). So you think that should be change= d to > > > > > e.g. just one descriptor for 4MB, right? > > > >=20 > > > > Limiting msize to the 9p transport device's maximum number of > > > > descriptors is conservative (i.e. 128 descriptors =3D 512 KB msize) > > > > because it doesn't take advantage of contiguous memory. I suggest > > > > leaving msize alone, adding a separate limit at which requests are = split > > > > according to the maximum descriptor chain length, and tweaking > > > > pack_sg_list() to coalesce adjacent pages. > > > >=20 > > > > That way msize can be large without necessarily using lots of > > > > descriptors (depending on the memory layout). > > >=20 > > > That was actually a tempting solution. Because it would neither requi= re > > > changes to the virtio specs (at least for a while) and it would also = work > > > with older QEMU versions. And for that pack_sg_list() portion of the = code > > > it would work well and easy as the buffer passed to pack_sg_list() is > > > contiguous already. > > >=20 > > > However I just realized for the zero-copy version of the code that wo= uld > > > be > > > more tricky. The ZC version already uses individual pages (struct pag= e, > > > hence PAGE_SIZE each) which are pinned, i.e. it uses pack_sg_list_p()= [1] > > > in combination with p9_get_mapped_pages() [2] > > >=20 > > > [1] > > > https://github.com/torvalds/linux/blob/7ddb58cb0ecae8e8b6181d736a8766= 7cc9 > > > ab8389/net/9p/trans_virtio.c#L218 [2] > > > https://github.com/torvalds/linux/blob/7ddb58cb0ecae8e8b6181d736a8766= 7cc9 > > > ab8389/net/9p/trans_virtio.c#L309 > > >=20 > > > So that would require much more work and code trying to sort and coal= esce > > > individual pages to contiguous physical memory for the sake of reduci= ng > > > virtio descriptors. And there is no guarantee that this is even possi= ble. > > > The kernel may simply return a non-contiguous set of pages which would > > > eventually end up exceeding the virtio descriptor limit again. > >=20 > > Order must be preserved so pages cannot be sorted by physical address. > > How about simply coalescing when pages are adjacent? >=20 > It would help, but not solve the issue we are talking about here: if 99% = of=20 > the cases could successfully merge descriptors to stay below the descript= or=20 > count limit, but in 1% of the cases it could not, then this still constru= es a=20 > severe runtime issue that could trigger at any time. >=20 > > > So looks like it was probably still easier and realistic to just add > > > virtio > > > capabilities for now for allowing to exceed current descriptor limit. > >=20 > > I'm still not sure why virtio-net, virtio-blk, virtio-fs, etc perform > > fine under today's limits while virtio-9p needs a much higher limit to > > achieve good performance. Maybe there is an issue in a layer above the > > vring that's causing the virtio-9p performance you've observed? >=20 > Are you referring to (somewhat) recent benchmarks when saying those would= all=20 > still perform fine today? I'm not referring to specific benchmark results. Just that none of those devices needed to raise the descriptor chain length, so I'm surprised that virtio-9p needs it because it's conceptually similar to these devices. > Vivek was running detailed benchmarks for virtiofs vs. 9p: > https://lists.gnu.org/archive/html/qemu-devel/2020-12/msg02704.html >=20 > For the virtio aspect discussed here, only the benchmark configurations= =20 > without cache are relevant (9p-none, vtfs-none) and under this aspect the= =20 > situation seems to be quite similar between 9p and virtio-fs. You'll also= note=20 > once DAX is enabled (vtfs-none-dax) that apparently boosts virtio-fs=20 > performance significantly, which however seems to corelate to numbers whe= n I=20 > am running 9p with msize > 300k. Note: Vivek was presumably running 9p=20 > effecively with msize=3D300k, as this was the kernel limitation at that t= ime. Agreed, virtio-9p and virtiofs are similar without caching. I think we shouldn't consider DAX here since it bypasses the virtqueue. > To bring things into relation: there are known performance aspects in 9p = that=20 > can be improved, yes, both on Linux kernel side and on 9p server side in = QEMU.=20 > For instance 9p server uses coroutines [1] and currently dispatches betwe= en=20 > worker thread(s) and main thread too often per request (partly addressed= =20 > already [2], but still WIP), which accumulates to overall latency. But Vi= vek=20 > was actually using a 9p patch here which disabled coroutines entirely, wh= ich=20 > suggests that the virtio transmission size limit still represents a=20 > bottleneck. These results were collected with 4k block size. Neither msize nor the descriptor chain length limits will be stressed, so I don't think these results are relevant here. Maybe a more relevant comparison would be virtio-9p, virtiofs, and virtio-blk when block size is large (e.g. 1M). The Linux block layer in the guest will split virtio-blk requests when they exceed the block queue limits. Stefan >=20 > [1] https://wiki.qemu.org/Documentation/9p#Coroutines > [2] https://wiki.qemu.org/Documentation/9p#Implementation_Plans >=20 > Best regards, > Christian Schoenebeck >=20 >=20 --eestfZp/R6WSRbB0 Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEhpWov9P5fNqsNXdanKSrs4Grc8gFAmGLmX4ACgkQnKSrs4Gr c8ib+Qf/SKVx+e+woQwfvKZrZAOngttbg1kfU9RmuRPfwP2+OWFJFTfGlZAQFDr0 fwZRAZV15JXseHW1CkEwe5SUq7doi6PaVdmL5FaNHxmKJ+XwdMRmq/TVMA2KA6w2 Ny+hxinVZzRN1MDknaB6rB6qv3w2X/kkxZvKF6s7iNDYEYzITfri29w7lK6NI+6V 0xOg7ivlUqKYTJ3JIHYq73F/imKmVUR34AFGDFKAFlmSuOiGjvqltmSm3GEwbWzb fZFp+nf0QqPG+FBtu9ixhZ7Svq2yjMy6eYYyrSRpIpOZWJNF/kP0ziSylaq2/oHp FoA/coxSaKhMvlM4s2/upEMtxQRT2Q== =ytbs -----END PGP SIGNATURE----- --eestfZp/R6WSRbB0--