Re: [PATCH v2 2/2] vhost: Add smp_rmb() in vhost_enable_notify()

[Gavin Shan <gshan@redhat.com>]

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