On 9/25/23 16:23, Stefan Hajnoczi wrote:
> On Fri, 25 Aug 2023 at 13:04, Ilya Maximets <i.maxim...@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.maxim...@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?
>
>> + }
>> +
>> + num_heads = vring_avail_idx(vq) - idx;
>>
>> /* Check it isn't doing very strange things with descriptor numbers. */
>> if (num_heads > vq->vring.num) {
>> --
>> 2.40.1
>>
>>
