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Tsirkin" To: Gavin Shan Cc: virtualization@lists.linux.dev, linux-kernel@vger.kernel.org, jasowang@redhat.com, davem@davemloft.net, stefanha@redhat.com, sgarzare@redhat.com, keirf@google.com, yihyu@redhat.com, shan.gavin@gmail.com Subject: Re: [PATCH v2 1/2] vhost: Add smp_rmb() in vhost_vq_avail_empty() Message-ID: <20240327075940-mutt-send-email-mst@kernel.org> References: <20240326233846.1086253-1-gshan@redhat.com> <20240326233846.1086253-2-gshan@redhat.com> Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: <20240326233846.1086253-2-gshan@redhat.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Wed, Mar 27, 2024 at 09:38:45AM +1000, Gavin Shan wrote: > A smp_rmb() has been missed in vhost_vq_avail_empty(), spotted by > Will Deacon . Otherwise, it's not ensured the > available ring entries pushed by guest can be observed by vhost > in time, leading to stale available ring entries fetched by vhost > in vhost_get_vq_desc(), as reported by Yihuang Yu on NVidia's > grace-hopper (ARM64) platform. > > /home/gavin/sandbox/qemu.main/build/qemu-system-aarch64 \ > -accel kvm -machine virt,gic-version=host -cpu host \ > -smp maxcpus=1,cpus=1,sockets=1,clusters=1,cores=1,threads=1 \ > -m 4096M,slots=16,maxmem=64G \ > -object memory-backend-ram,id=mem0,size=4096M \ > : \ > -netdev tap,id=vnet0,vhost=true \ > -device virtio-net-pci,bus=pcie.8,netdev=vnet0,mac=52:54:00:f1:26:b0 > : > guest# netperf -H 10.26.1.81 -l 60 -C -c -t UDP_STREAM > virtio_net virtio0: output.0:id 100 is not a head! > > Add the missed smp_rmb() in vhost_vq_avail_empty(). Note that it > should be safe until vq->avail_idx is changed by commit 275bf960ac697 > ("vhost: better detection of available buffers"). > > Fixes: 275bf960ac697 ("vhost: better detection of available buffers") > Cc: # v4.11+ > Reported-by: Yihuang Yu > Signed-off-by: Gavin Shan > --- > drivers/vhost/vhost.c | 11 ++++++++++- > 1 file changed, 10 insertions(+), 1 deletion(-) > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > index 045f666b4f12..00445ab172b3 100644 > --- a/drivers/vhost/vhost.c > +++ b/drivers/vhost/vhost.c > @@ -2799,9 +2799,18 @@ bool vhost_vq_avail_empty(struct vhost_dev *dev, struct vhost_virtqueue *vq) > r = vhost_get_avail_idx(vq, &avail_idx); > if (unlikely(r)) > return false; > + > vq->avail_idx = vhost16_to_cpu(vq, avail_idx); > + if (vq->avail_idx != vq->last_avail_idx) { > + /* Similar to what's done in vhost_get_vq_desc(), we need > + * to ensure the available ring entries have been exposed > + * by guest. > + */ A slightly clearer comment: /* Since we have updated avail_idx, the following call to * vhost_get_vq_desc will read available ring entries. * Make sure that read happens after the avail_idx read. */ Pls repost with that, and I will apply. Also add suggested-by for will. > + smp_rmb(); > + return false; > + } > > - return vq->avail_idx == vq->last_avail_idx; > + return true; > } > EXPORT_SYMBOL_GPL(vhost_vq_avail_empty); As a follow-up patch, we should clean out code duplication that accumulated with 3 places reading avail idx in essentially the same way - this duplication is what causes the mess in the 1st place. > -- > 2.44.0