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Date: Thu, 28 Mar 2024 10:27:33 +1000
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Subject: Re: [PATCH v2 1/2] vhost: Add smp_rmb() in vhost_vq_avail_empty()
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To: Jason Wang <jasowang@redhat.com>
Cc: virtualization@lists.linux.dev, linux-kernel@vger.kernel.org,
 mst@redhat.com, davem@davemloft.net, stefanha@redhat.com,
 sgarzare@redhat.com, keirf@google.com, yihyu@redhat.com,
 shan.gavin@gmail.com, Will Deacon <will@kernel.org>
References: <20240326233846.1086253-1-gshan@redhat.com>
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From: Gavin Shan <gshan@redhat.com>
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On 3/27/24 17:42, Jason Wang wrote:
> On Wed, Mar 27, 2024 at 3:35 PM Gavin Shan <gshan@redhat.com> wrote:
>>
>> On 3/27/24 14:08, Gavin Shan wrote:
>>> On 3/27/24 12:44, Jason Wang wrote:
>>>> On Wed, Mar 27, 2024 at 10:34 AM Jason Wang <jasowang@redhat.com> wrote:
>>>>> On Wed, Mar 27, 2024 at 7:39 AM Gavin Shan <gshan@redhat.com> 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.
>>>>>> +                */
>>>>>
>>>>> We need to be more verbose here. For example, which load needs to be
>>>>> ordered with which load.
>>>>>
>>>>> The rmb in vhost_get_vq_desc() is used to order the load of avail idx
>>>>> and the load of head. It is paired with e.g virtio_wmb() in
>>>>> virtqueue_add_split().
>>>>>
>>>>> vhost_vq_avail_empty() are mostly used as a hint in
>>>>> vhost_net_busy_poll() which is under the protection of the vq mutex.
>>>>>
>>>>> An exception is the tx_can_batch(), but in that case it doesn't even
>>>>> want to read the head.
>>>>
>>>> Ok, if it is needed only in that path, maybe we can move the barriers there.
>>>>
>>>
>>> [cc Will Deacon]
>>>
>>> Jason, appreciate for your review and comments. I think PATCH[1/2] is
>>> the fix for the hypothesis, meaning PATCH[2/2] is the real fix. However,
>>> it would be nice to fix all of them in one shoot. I will try with PATCH[2/2]
>>> only to see if our issue will disappear or not. However, the issue still
>>> exists if PATCH[2/2] is missed.
>>>
>>
>> Jason, PATCH[2/2] is sufficient to fix our current issue. I tried with PATCH[2/2]
>> only and unable to hit the issue. However, PATCH[1/2] may be needed by other scenarios.
>> So it would be nice to fix them in one shoot.
> 
> Yes, see below.
> 
>>
>>
>>> Firstly, We were failing on the transmit queue and {tvq, rvq}->busyloop_timeout
>>> == false if I remember correctly. So the added smp_rmb() in vhost_vq_avail_empty()
>>> is only a concern to tx_can_batch(). A mutex isn't enough to ensure the order
>>> for the available index and available ring entry (head). For example, vhost_vq_avail_empty()
>>> called by tx_can_batch() can see next available index, but its corresponding
>>> available ring entry (head) may not be seen by vhost yet if smp_rmb() is missed.
>>> The next call to get_tx_bufs(), where the available ring entry (head) doesn't
>>> arrived yet, leading to stale available ring entry (head) being fetched.
>>>
>>>     handle_tx_copy
>>>       get_tx_bufs                 // smp_rmb() won't be executed when vq->avail_idx != vq->last_avail_idx
>>>       tx_can_batch
>>>         vhost_vq_avail_empty      // vq->avail_idx is updated from vq->avail->idx
>>>
>>> The reason why I added smp_rmb() to vhost_vq_avail_empty() is because the function
>>> is a exposed API, even it's only used by drivers/vhost/net.c at present. It means
>>> the API has been broken internally. So it seems more appropriate to fix it up in
>>> vhost_vq_avail_empty() so that the API's users needn't worry about the memory access
>>> order.
> 
> When tx_can_batch returns true it means there's still pending tx
> buffers. Since it might read indices so it still can bypass the
> smp_rmb() in the vhost_get_vq_desc().
> 
> I'd suggest adding those above to change log.
> 
> With this,
> 
> Acked-by: Jason Wang <jasowang@redhat.com>
> 

Thanks, Jason. The change log has been improved as you suggested in
v3, which was posted for further review.

Thanks,
Gavin

>>>
>>>>>
>>>>>
>>>>>> +               smp_rmb();
>>>>>> +               return false;
>>>>>> +       }
>>>>>>
>>>>>> -       return vq->avail_idx == vq->last_avail_idx;
>>>>>> +       return true;
>>>>>>    }
>>>>>>    EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);