Re: [PATCH v2 1/2] vhost: Add smp_rmb() in vhost_vq_avail_empty()

[Gavin Shan <gshan@redhat.com>]

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 <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_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: <stable@kernel.org> # v4.11+
>> 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 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