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=-5.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 11BE3C433E0 for ; Fri, 29 May 2020 14:30:50 +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 CFA3A2073B for ; Fri, 29 May 2020 14:30:49 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="QVYMUMOF" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org CFA3A2073B 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]:48496 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jeg24-0002w9-WE for qemu-devel@archiver.kernel.org; Fri, 29 May 2020 10:30:49 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:60976) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jeg0v-0000vm-Jv for qemu-devel@nongnu.org; Fri, 29 May 2020 10:29:37 -0400 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:37706 helo=us-smtp-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1jeg0t-0004W8-V5 for qemu-devel@nongnu.org; Fri, 29 May 2020 10:29:36 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1590762573; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=R/NaKJVknJO/XeQRWduHilWywmJ+oqjC3hC94DHB3oM=; b=QVYMUMOFk6a4Z9kCLCr4NLBm1NE2pfhQg3RR/eJzCxKeMaRaix1pC75/Y0PWEKB7gCcb0f dShBj1zR4zvYlhfhD280ba8Yve5zM3q0tZv7VfSKFcOAepX5J6+8xOJaFHc8H1wQgDFIXb HY8my2Fs6hjDQ+A8EpVh377jDRnyO40= 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-378-djoVszw3NLKD0Oz4JActcQ-1; Fri, 29 May 2020 10:29:32 -0400 X-MC-Unique: djoVszw3NLKD0Oz4JActcQ-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 12E688014D4; Fri, 29 May 2020 14:29:31 +0000 (UTC) Received: from work-vm (ovpn-113-111.ams2.redhat.com [10.36.113.111]) by smtp.corp.redhat.com (Postfix) with ESMTPS id E667F99DE6; Fri, 29 May 2020 14:29:16 +0000 (UTC) Date: Fri, 29 May 2020 15:29:13 +0100 From: "Dr. David Alan Gilbert" To: Stefan Hajnoczi Subject: Re: [PATCH 2/5] vhost: involve device backends in feature negotiation Message-ID: <20200529142913.GM2856@work-vm> References: <20200522171726.648279-1-stefanha@redhat.com> <20200522171726.648279-3-stefanha@redhat.com> <20200529135641.GC367530@stefanha-x1.localdomain> MIME-Version: 1.0 In-Reply-To: <20200529135641.GC367530@stefanha-x1.localdomain> User-Agent: Mutt/1.13.4 (2020-02-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Disposition: inline Received-SPF: pass client-ip=205.139.110.120; envelope-from=dgilbert@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/05/28 23:43:13 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 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_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_PASS=-0.001, T_HK_NAME_DR=0.01 autolearn=_AUTOLEARN 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: Kevin Wolf , Laurent Vivier , Thomas Huth , Eduardo Habkost , qemu-block@nongnu.org, "Michael S. Tsirkin" , Jason Wang , cohuck@redhat.com, qemu-devel@nongnu.org, Max Reitz , "Gonglei \(Arei\)" , Gerd Hoffmann , Paolo Bonzini , =?iso-8859-1?Q?Marc-Andr=E9?= Lureau , Fam Zheng , Raphael Norwitz Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Stefan Hajnoczi (stefanha@redhat.com) wrote: > On Fri, May 29, 2020 at 03:04:54PM +0800, Jason Wang wrote: > > > > On 2020/5/23 上午1:17, Stefan Hajnoczi wrote: > > > Many vhost devices in QEMU currently do not involve the device backend > > > in feature negotiation. This seems fine at first glance for device types > > > without their own feature bits (virtio-net has many but other device > > > types have none). > > > > > > This overlooks the fact that QEMU's virtqueue implementation and the > > > device backend's implementation may support different features. QEMU > > > must not report features to the guest that the the device backend > > > doesn't support. > > > > > > For example, QEMU supports VIRTIO 1.1 packed virtqueues while many > > > existing vhost device backends do not. When the user sets packed=on the > > > device backend breaks. This should have been handled gracefully by > > > feature negotiation instead. > > > > > > Introduce vhost_get_default_features() and update all vhost devices in > > > QEMU to involve the device backend in feature negotiation. > > > > > > This patch fixes the following error: > > > > > > $ x86_64-softmmu/qemu-system-x86_64 \ > > > -drive if=virtio,file=test.img,format=raw \ > > > -chardev socket,path=/tmp/vhost-user-blk.sock,id=char0 \ > > > -device vhost-user-blk-pci,chardev=char0,packed=on \ > > > -object memory-backend-memfd,size=1G,share=on,id=ram0 \ > > > -M accel=kvm,memory-backend=ram0 > > > qemu-system-x86_64: Failed to set msg fds. > > > qemu-system-x86_64: vhost VQ 0 ring restore failed: -1: Success (0) > > > > > > It looks to me this could be fixed simply by adding VIRTIO_F_RING_PACKED > > into user_feature_bits in vhost-user-blk.c. > > > > And for the rest, we need require them to call vhost_get_features() with the > > proper feature bits that needs to be acked by the backend. > > There is a backwards-compatibility problem: we cannot call > vhost_get_features() with the full set of feature bits that each device > type supports because existing vhost-user backends don't advertise > features properly. QEMU disables features not advertised by the > vhost-user backend. > > For example, if we add VIRTIO_RING_F_EVENT_IDX then it will always be > disabled for existing libvhost-user backends because they don't > advertise this bit :(. > > The reason I introduced vhost_get_default_features() is to at least > salvage VIRTIO_F_IOMMU_PLATFORM and VIRTIO_F_RING_PACKED. These bits can > be safely negotiated for all devices. > > Any new transport/vring VIRTIO feature bits can also be added to > vhost_get_default_features() safely. > > If a vhost-user device wants to use device type-specific feature bits > then it really needs to call vhost_get_features() directly as you > suggest. But it's important to check whether existing vhost-user > backends actually advertise them - because if they don't advertise them > but rely on them then we'll break existing backends. What about adding a VHOST_USER_PROTOCOL_F_BACKEND_F to indicate the backend knows how to advertise the backend features. (Although my understanding of virtio feature flags is thin) Dave > Would you prefer a different approach? > > > > diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c > > > index aff98a0ede..f8a144dcd0 100644 > > > --- a/hw/virtio/vhost.c > > > +++ b/hw/virtio/vhost.c > > > @@ -48,6 +48,23 @@ static unsigned int used_memslots; > > > static QLIST_HEAD(, vhost_dev) vhost_devices = > > > QLIST_HEAD_INITIALIZER(vhost_devices); > > > +/* > > > + * Feature bits that device backends must explicitly report. Feature bits not > > > + * listed here maybe set by QEMU without checking with the device backend. > > > + * Ideally all feature bits would be listed here but existing vhost device > > > + * implementations do not explicitly report bits like VIRTIO_F_VERSION_1, so we > > > + * can only assume they are supported. > > > > > > For most backends, we care about the features for datapath. So this is not > > true at least for networking device, since VIRTIO_F_VERSION_1 have impact on > > the length of vnet header length. > > The net device is in good shape and doesn't use vhost_default_feature_bits[]. > > vhost_default_feature_bits[] is for devices that haven't been > negotiating feature bits properly. The goal is start involving them in > feature negotiation without breaking existing backends. > > Would you like me to rephrase this comment in some way? > > > > + * > > > + * New feature bits added to the VIRTIO spec should usually be included here > > > + * so that existing vhost device backends that do not support them yet continue > > > + * to work. > > > > > > It actually depends on the type of backend. > > > > Kernel vhost-net does not validate GSO features since qemu can talk directly > > to TAP and vhost doesn't report those features. But for vhost-user GSO > > features must be validated by qemu since qemu can't see what is behind > > vhost-user. > > Maybe the comment should say "New transport/vring feature bits". I'm > thinking about things like VIRTIO_F_RING_PACKED that are not > device-specific but require backend support. > > The GSO features you mentioned are device-specific. Devices that want to > let the backend advertise device-specific features cannot use > vhost_default_feature_bits[]. > > > > + */ > > > +static const int vhost_default_feature_bits[] = { > > > + VIRTIO_F_IOMMU_PLATFORM, > > > + VIRTIO_F_RING_PACKED, > > > + VHOST_INVALID_FEATURE_BIT > > > +}; > > > + > > > bool vhost_has_free_slot(void) > > > { > > > unsigned int slots_limit = ~0U; > > > @@ -1468,6 +1485,11 @@ uint64_t vhost_get_features(struct vhost_dev *hdev, const int *feature_bits, > > > return features; > > > } > > > +uint64_t vhost_get_default_features(struct vhost_dev *hdev, uint64_t features) > > > +{ > > > + return vhost_get_features(hdev, vhost_default_feature_bits, features); > > > +} > > > > > > There's probably no need for a new helper, we can do all these inside > > vhost_get_features(). > > Would you prefer: > > extern const int vhost_default_feature_bits[]; > > And then callers do: > > vhost_get_features(hdev, vhost_default_feature_bits, features); > > ? -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK