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 11BCEC433F5 for ; Fri, 12 Nov 2021 15:56:25 +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 6154560F45 for ; Fri, 12 Nov 2021 15:56:24 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 6154560F45 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]:40930 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1mlYud-0006Nj-5d for qemu-devel@archiver.kernel.org; Fri, 12 Nov 2021 10:56:23 -0500 Received: from eggs.gnu.org ([209.51.188.92]:39836) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mlYtb-0005Gz-Rw for qemu-devel@nongnu.org; Fri, 12 Nov 2021 10:55:19 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:31981) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mlYtZ-0004mp-CD for qemu-devel@nongnu.org; Fri, 12 Nov 2021 10:55:18 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1636732516; 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=+760LTtoGvK9S9T3GKPAud3xCXdNFrISENCZFhQLwA4=; b=e6+jjRhXBKW5WEL4+OXPIMStZSSHgImBOnIuZy8Xo9Bmqy8QsjsVcF+xeu0DMFgA2U1xWG xvHSvxdIdYiXej9Xm+5leuBXtsXQyhScZmyE1t9+GNU7B+aUzeQ4sw7R41PbJfZ7HyzZxz cwdga5fCreNGe5iFkMVqh1hXZbpahqU= 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-493-e-1yC0azOX6JnkoxNey93w-1; Fri, 12 Nov 2021 10:55:13 -0500 X-MC-Unique: e-1yC0azOX6JnkoxNey93w-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 3F17C101AFA7; Fri, 12 Nov 2021 15:55:12 +0000 (UTC) Received: from localhost (unknown [10.39.194.90]) by smtp.corp.redhat.com (Postfix) with ESMTPS id B55C35D9DC; Fri, 12 Nov 2021 15:55:11 +0000 (UTC) From: Cornelia Huck To: Halil Pasic Subject: Re: [RFC PATCH v1 1/3] virtio: introduce virtio_force_modern() In-Reply-To: <20211112164208.71a2ca73.pasic@linux.ibm.com> Organization: Red Hat GmbH References: <20211028220017.930806-1-pasic@linux.ibm.com> <20211028220017.930806-2-pasic@linux.ibm.com> <87tugzc26y.fsf@redhat.com> <20211112164208.71a2ca73.pasic@linux.ibm.com> User-Agent: Notmuch/0.33.1 (https://notmuchmail.org) Date: Fri, 12 Nov 2021 16:55:10 +0100 Message-ID: <874k8hmkrl.fsf@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=cohuck@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Received-SPF: pass client-ip=216.205.24.124; envelope-from=cohuck@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: Thomas Huth , "Michael S. Tsirkin" , David Hildenbrand , Richard Henderson , qemu-devel@nongnu.org, Halil Pasic , Christian Borntraeger , qemu-s390x@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, Nov 12 2021, Halil Pasic wrote: > On Fri, 29 Oct 2021 16:53:25 +0200 > Cornelia Huck wrote: > >> On Fri, Oct 29 2021, Halil Pasic wrote: >> >> > Legacy vs modern should be detected via transport specific means. We >> > can't wait till feature negotiation is done. Let us introduce >> > virtio_force_modern() as a means for the transport code to signal >> > that the device should operate in modern mode (because a modern driver >> > was detected). >> > >> > Signed-off-by: Halil Pasic >> > --- >> > >> > I'm still struggling with how to deal with vhost-user and co. The >> > problem is that I'm not very familiar with the life-cycle of, let us >> > say, a vhost_user device. >> > >> > Looks to me like the vhost part might be just an implementation detail, >> > and could even become a hot swappable thing. >> > >> > Another thing is, that vhost processes set_features differently. It >> > might or might not be a good idea to change this. >> > >> > Does anybody know why don't we propagate the features on features_set, >> > but under a set of different conditions, one of which is the vhost >> > device is started? >> > --- >> > hw/virtio/virtio.c | 12 ++++++++++++ >> > include/hw/virtio/virtio.h | 1 + >> > 2 files changed, 13 insertions(+) >> > >> > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c >> > index 3a1f6c520c..75aee0e098 100644 >> > --- a/hw/virtio/virtio.c >> > +++ b/hw/virtio/virtio.c >> > @@ -3281,6 +3281,18 @@ void virtio_init(VirtIODevice *vdev, const char *name, >> > vdev->use_guest_notifier_mask = true; >> > } >> > >> > +void virtio_force_modern(VirtIODevice *vdev) >> >> I'm not sure I like that name. We're not actually forcing the >> device to be modern, we just set an early indication in the device >> before proper feature negotiation has finished. Maybe >> virtio_indicate_modern()? > > > I don't like virtio_indicate_modern(dev) form object orientation > perspective. In an OO language one would write it like > dev.virtio_indicate_modern() > which would read like the device should indicate modern to somebody. I think that is actually what happens: we indicate that it is a modern device to the code making the endianness decisions. > > In my opinion what happens is that we want to disable the legacy > interface if it is exposed by the device, or in other words instruct the > device that should act (precisely and exclusively) according to the > interface specification of the modern interface. I don't see us disabling anything; the driver has already chosen what they want, and we simply need to make sure that all code honours that decision. > > Maybe we can find a better name than force_modern, but I don't think > indicate_modern is a better name. > >> >> > +{ >> > + /* >> > + * This takes care of the devices that implement config space access >> > + * in QEMU. For vhost-user and similar we need to make sure the features >> > + * are actually propagated to the device implementing the config space. >> > + * >> > + * A VirtioDeviceClass callback may be a good idea. >> > + */ >> > + virtio_set_features(vdev, (1ULL << VIRTIO_F_VERSION_1)); >> >> Do we really need/want to do the whole song-and-dance for setting >> features, just for setting VERSION_1? > > When doing the whole song-and-dance the chance is higher that the > information will propagate to every place it needs to reach. For > example to the acked_features of vhost_dev. I've just posted a v2 RFC. > It should not be hard to see what I mean after examining that RFC. > >> Devices may modify some of their >> behaviour or features, depending on what features they are called with, > > I believe, if this is the case, we want the behavior that corresponds to > VERSION_1 set, i.e. 'modern'. So in my understanding this is rather good > than bad. > >> and we will be calling this one again later with what is likely a >> different feature set. > > That is true, but the driver is allowed to set the features multiple > times, and since transports only support piecemeal access to the > features (32 bits at a time), I guess this is biz as usual. Also see my comment in the v2: I'm not sure how well tested that actually is. > >>Also, the return code is not checked. >> > > That is true! It might be a good idea to log an error. Unfortunately I > don't think there is anything else we can sanely do. > >> Maybe introduce a new function that sets guest_features directly and >> errors out if the features are not set in host_features? > > See above. > >> If we try to >> set VERSION_1 here despite the device not offering it, we are in a >> pickle anyway, as we should not have gotten here if we did not offer it, >> and we really should moan and fail in that case. > > I agree about the moan part. I'm not sure what is the best way to > 'fail'. Maybe we should continue this discussion in the v2 thread. Yeah, let's continue there, since that code is a bit different.