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Tsirkin" 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 References: <20240326233846.1086253-1-gshan@redhat.com> <20240326233846.1086253-2-gshan@redhat.com> <20240327075940-mutt-send-email-mst@kernel.org> From: Gavin Shan In-Reply-To: <20240327075940-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 3/27/24 22:07, Michael S. Tsirkin wrote: > 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. > Sure, the suggested comments have been included to v3. > >> + 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. > Yes, nice idea. I've added PATCH[v3 3/3] to improve vhost_get_avail_idx() to handle the memory barrier since all the callers have the concern. v3: https://lore.kernel.org/virtualization/20240328002149.1141302-1-gshan@redhat.com/ Thanks, Gavin