Messages in this thread | | | Date | Wed, 27 Mar 2024 14:10:37 +1000 | Subject | Re: [PATCH v2 2/2] vhost: Add smp_rmb() in vhost_enable_notify() | From | Gavin Shan <> |
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On 3/27/24 12:41, Jason Wang wrote: > On Wed, Mar 27, 2024 at 7:39 AM Gavin Shan <gshan@redhat.com> wrote: >> >> A smp_rmb() has been missed in vhost_enable_notify(), inspired by >> Will Deacon <will@kernel.org>. 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_enable_notify(). Note that it >> should be safe until vq->avail_idx is changed by commit d3bb267bbdcb >> ("vhost: cache avail index in vhost_enable_notify()"). >> >> Fixes: d3bb267bbdcb ("vhost: cache avail index in vhost_enable_notify()") >> Cc: <stable@kernel.org> # v5.18+ >> Reported-by: Yihuang Yu <yihyu@redhat.com> >> Signed-off-by: Gavin Shan <gshan@redhat.com> >> --- >> 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 00445ab172b3..58f9d6a435f0 100644 >> --- a/drivers/vhost/vhost.c >> +++ b/drivers/vhost/vhost.c >> @@ -2847,9 +2847,18 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq) >> &vq->avail->idx, 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. >> + */ >> + smp_rmb(); >> + return true; >> + } >> >> - return vq->avail_idx != vq->last_avail_idx; >> + return false; > > So we only care about the case when vhost_enable_notify() returns true. > > In that case, I think you want to order with vhost_get_vq_desc(): > > last_avail_idx = vq->last_avail_idx; > > if (vq->avail_idx == vq->last_avail_idx) { /* false */ > } > > vhost_get_avail_head(vq, &ring_head, last_avail_idx) > > Assuming I understand the patch correctly. > > Acked-by: Jason Wang <jasowang@redhat.com> >
Jason, thanks for your review and comments. Your understanding is exactly what I understood.
> >> } >> EXPORT_SYMBOL_GPL(vhost_enable_notify); >>
Thanks, Gavin
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