Re: [PATCH] virtio: use shadow_avail_idx while checking number of heads

[Ilya Maximets]

On 9/25/23 17:12, Stefan Hajnoczi wrote:
> On Mon, 25 Sept 2023 at 11:02, Ilya Maximets <i.maximets@ovn.org> wrote:
>>
>> On 9/25/23 16:23, Stefan Hajnoczi wrote:
>>> On Fri, 25 Aug 2023 at 13:04, Ilya Maximets <i.maximets@ovn.org> wrote:
>>>>
>>>> We do not need the most up to date number of heads, we only want to
>>>> know if there is at least one.
>>>>
>>>> Use shadow variable as long as it is not equal to the last available
>>>> index checked.  This avoids expensive qatomic dereference of the
>>>> RCU-protected memory region cache as well as the memory access itself
>>>> and the subsequent memory barrier.
>>>>
>>>> The change improves performance of the af-xdp network backend by 2-3%.
>>>>
>>>> Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
>>>> ---
>>>>  hw/virtio/virtio.c | 10 +++++++++-
>>>>  1 file changed, 9 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
>>>> index 309038fd46..04bf7cc977 100644
>>>> --- a/hw/virtio/virtio.c
>>>> +++ b/hw/virtio/virtio.c
>>>> @@ -999,7 +999,15 @@ void virtqueue_push(VirtQueue *vq, const 
>>>> VirtQueueElement *elem,
>>>>  /* Called within rcu_read_lock().  */
>>>>  static int virtqueue_num_heads(VirtQueue *vq, unsigned int idx)
>>>>  {
>>>> -    uint16_t num_heads = vring_avail_idx(vq) - idx;
>>>> +    uint16_t num_heads;
>>>> +
>>>> +    if (vq->shadow_avail_idx != idx) {
>>>> +        num_heads = vq->shadow_avail_idx - idx;
>>>> +
>>>> +        return num_heads;
>>>
>>> This still needs to check num_heads > vq->vring.num and return -EINVAL
>>> as is done below.
>>
>> Hmm, yeas, you're right.  If the value was incorrect initially, the shadow
>> will be incorrect.  However, I think we should just not return here in this
>> case and let vring_avail_idx() to grab an actual new value below.  Otherwise
>> we may never break out of this error.
>>
>> Does that make sense?
> 
> No, because virtio_error() marks the device as broken. The device
> requires a reset in order to function again. Fetching
> vring_avail_idx() again won't help.

OK, I see.  In this case we're talking about situation where
vring_avail_idx() was called in some other place and stored a bad value
in the shadow variable, then virtqueue_num_heads() got called.  Right?

AFAIU, we can still just fall through here and let vring_avail_idx()
to read the index again and fail the existing check.  That would happen
today without this patch applied.

I'm jut trying to avoid duplication of the virtio_error call, i.e.:

    if (vq->shadow_avail_idx != idx) {
        num_heads = vq->shadow_avail_idx - idx;

        /* Check it isn't doing very strange things with descriptor numbers. */
        if (num_heads > vq->vring.num) {
            virtio_error(vq->vdev, "Guest moved used index from %u to %u",
                         idx, vq->shadow_avail_idx);
            return -EINVAL;
        }
        return num_heads;
    }

vs

    if (vq->shadow_avail_idx != idx) {
        num_heads = vq->shadow_avail_idx - idx;

        /* Only use the shadow value if it was good initially. */
        if (num_heads <= vq->vring.num) {
            return num_heads;
        }
    }


What do you think?

Best regards, Ilya Maximets.