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