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 X-Spam-Level: X-Spam-Status: No, score=-6.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5E320C433DB for ; Mon, 22 Mar 2021 10:53:34 +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 0238361983 for ; Mon, 22 Mar 2021 10:53:33 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0238361983 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:54476 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lOIBh-0003py-3T for qemu-devel@archiver.kernel.org; Mon, 22 Mar 2021 06:53:33 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:40096) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lOI9u-0002bR-Jk for qemu-devel@nongnu.org; Mon, 22 Mar 2021 06:51:42 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:50009) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1lOI9s-0002us-KK for qemu-devel@nongnu.org; Mon, 22 Mar 2021 06:51:42 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1616410299; 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=VEy/Ye4i5eRJcFSJCAJSKiatYNBBfFwAO5ms8tBEIdo=; b=Z7vG2JjTk5w83HVEDnlQ86Djyib1Dn6B3yyi6O1EA0W3zpFPHjHrfb/hKHDYmdrjcVrPKT Fi96xNzwNDVKNvXswgA6sQzKUJr50lMT6TK8GmxMxcuTQwZY4Jwc+R0OQOTb47gAeRrSiV yF+ZdBR+G3OMRMwCIeZvgeQ8qXEYFj8= 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-483-Mai2JD7uMkiuh6sqUZ16JA-1; Mon, 22 Mar 2021 06:51:37 -0400 X-MC-Unique: Mai2JD7uMkiuh6sqUZ16JA-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 5F2E78189C6 for ; Mon, 22 Mar 2021 10:51:36 +0000 (UTC) Received: from localhost (ovpn-114-89.ams2.redhat.com [10.36.114.89]) by smtp.corp.redhat.com (Postfix) with ESMTP id 05C8160BE5; Mon, 22 Mar 2021 10:51:35 +0000 (UTC) Date: Mon, 22 Mar 2021 10:51:34 +0000 From: Stefan Hajnoczi To: Eugenio Perez Martin Subject: Re: [RFC PATCH 13/27] vhost: Send buffers to device Message-ID: References: <20201120185105.279030-1-eperezma@redhat.com> <20201120185105.279030-14-eperezma@redhat.com> <20201208081621.GR203660@stefanha-x1.localdomain> <20201210115547.GH416119@stefanha-x1.localdomain> MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 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="Hax29pa3SUMeUY64" Content-Disposition: inline Received-SPF: pass client-ip=216.205.24.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, 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_H4=-0.01, RCVD_IN_MSPIKE_WL=-0.01, 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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" --Hax29pa3SUMeUY64 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Thu, Mar 11, 2021 at 07:53:53PM +0100, Eugenio Perez Martin wrote: > On Fri, Jan 22, 2021 at 7:18 PM Eugenio Perez Martin > wrote: > > > > On Thu, Dec 10, 2020 at 12:55 PM Stefan Hajnoczi = wrote: > > > > > > On Wed, Dec 09, 2020 at 07:41:23PM +0100, Eugenio Perez Martin wrote: > > > > On Tue, Dec 8, 2020 at 9:16 AM Stefan Hajnoczi = wrote: > > > > > On Fri, Nov 20, 2020 at 07:50:51PM +0100, Eugenio P=E9rez wrote: > > > > > > + while (true) { > > > > > > + int r; > > > > > > + if (virtio_queue_full(vq)) { > > > > > > + break; > > > > > > + } > > > > > > > > > > Why is this check necessary? The guest cannot provide more descri= ptors > > > > > than there is ring space. If that happens somehow then it's a dri= ver > > > > > error that is already reported in virtqueue_pop() below. > > > > > > > > > > > > > It's just checked because virtqueue_pop prints an error on that cas= e, > > > > and there is no way to tell the difference between a regular error = and > > > > another caused by other causes. Maybe the right thing to do is just= to > > > > not to print that error? Caller should do the error printing in tha= t > > > > case. Should we return an error code? > > > > > > The reason an error is printed today is because it's a guest error th= at > > > never happens with correct guest drivers. Something is broken and the > > > user should know about it. > > > > > > Why is "virtio_queue_full" (I already forgot what that actually means= , > > > it's not clear whether this is referring to avail elements or used > > > elements) a condition that should be silently ignored in shadow vqs? > > > > > > > TL;DR: It can be changed to a check of the number of available > > descriptors in shadow vq, instead of returning as a regular operation. > > However, I think that making it a special return of virtqueue_pop > > could help in devices that run to completion, avoiding having to > > duplicate the count logic in them. > > > > The function virtio_queue_empty checks if the vq has all descriptors > > available, so the device has no more work to do until the driver makes > > another descriptor available. I can see how it can be a bad name > > choice, but virtio_queue_full means the opposite: device has pop() > > every descriptor available, and it has not returned any, so the driver > > cannot progress until the device marks some descriptors as used. > > > > As I understand, if vq->in_use >vq->num would mean we have a bug in > > the device vq code, not in the driver. virtio_queue_full could even be > > changed to "assert(vq->inuse <=3D vq->vring.num); return vq->inuse =3D= =3D > > vq->vring.num", as long as vq->in_use is operated right. > > > > If we hit vq->in_use =3D=3D vq->num in virtqueue_pop it means the devic= e > > tried to pop() one more buffer after having all of them available and > > pop'ed. This would be invalid if the device is counting right the > > number of in_use descriptors, but then we are duplicating that logic > > in the device and the vq. Devices call virtqueue_pop() until it returns NULL. They don't need to count virtqueue buffers explicitly. It returns NULL when vq->num virtqueue buffers have already been popped (either because virtio_queue_empty() is true or because an invalid driver state is detected by checking vq->num in virtqueue_pop()). > > In shadow vq this situation happens with the correct guest network > > driver, since the rx queue is filled for the device to write. Network > > device in qemu fetch descriptors on demand, but shadow vq fetch all > > available in batching. If the driver just happens to fill the queue of > > available descriptors, the log will raise, so we need to check in > > handle_sw_lm_vq before calling pop(). Of course the shadow vq can > > duplicate guest_vq->in_use instead of checking virtio_queue_full, but > > then it needs to check two things for every virtqueue_pop() [1]. I don't understand this scenario. It sounds like you are saying the guest and shadow rx vq are not in sync so there is a case where vq->in_use > vq->num is triggered? I'm not sure how that can happen since both vqs have equal vq->num. Maybe you can explain the scenario in more detail? Stefan --Hax29pa3SUMeUY64 Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEhpWov9P5fNqsNXdanKSrs4Grc8gFAmBYdrYACgkQnKSrs4Gr c8ivXggAsJm/hTj2ycBiS00G7qqTkGc+T9Y1fPQXmZvlUHDWb8XmwjmF2U8mPta+ DW3Mjl4a3YYETHrmLZFwCpf/TJwlgv4MZgY1khLiTT+5XO7wAecYPn1aoqeh5src LSkh6SxAj6H3bx+dZDHPbBAuWo4pNMPZGZokXjOZqmPnDzEro6lUxhQ7zmtrpz1u ttEpRVVMgzCTXlFN+QWW0yOWtUyTCFgTPmsw3TBk7/gbpSTEIHIHfGqk5tB1yb+Y LcqZBwnnXnPyZ/Z+PBZaF0CtXeUR257zLmH1tymcBhnaEuoaZuXcXb5UdMvtwTf0 2NQCUB1RcrINauyz2JvQnwJA1Tr5lQ== =e2w5 -----END PGP SIGNATURE----- --Hax29pa3SUMeUY64--