[PATCHv2] virtio: limit avail bytes lookahead

Commit 0d8d7690850eb0cf2b2b60933cf47669a6b6f18f introduced
a regression in virtio-net performance because it looks
into the ring aggressively while we really only care
about a single packet worth of buffers.
Reported as bugzilla 1066055 in launchpad.

To fix, add parameters limiting lookahead, and
use in virtqueue_avail_bytes.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reported-by: Edivaldo de Araujo Pereira <edivaldoapereira@yahoo.com.br>
Tested-by: Edivaldo de Araujo Pereira <edivaldoapereira@yahoo.com.br>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
---

Anthony says he wants to test this, I only compiled this patch.

 hw/virtio-rng.c        | 12 +++++++++---
 hw/virtio-serial-bus.c |  2 +-
 hw/virtio.c            | 15 ++++++++-------
 hw/virtio.h            |  3 ++-
 4 files changed, 20 insertions(+), 12 deletions(-)

"Michael S. Tsirkin" <mst@redhat.com> writes:

> Commit 0d8d7690850eb0cf2b2b60933cf47669a6b6f18f introduced
> a regression in virtio-net performance because it looks
> into the ring aggressively while we really only care
> about a single packet worth of buffers.
> Reported as bugzilla 1066055 in launchpad.
>
> To fix, add parameters limiting lookahead, and
> use in virtqueue_avail_bytes.
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> Reported-by: Edivaldo de Araujo Pereira <edivaldoapereira@yahoo.com.br>
> Tested-by: Edivaldo de Araujo Pereira <edivaldoapereira@yahoo.com.br>
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>
> Anthony says he wants to test this, I only compiled this patch.

I tested it and it works.

Applied. Thanks.

Regards,

Anthony Liguori

>
>  hw/virtio-rng.c        | 12 +++++++++---
>  hw/virtio-serial-bus.c |  2 +-
>  hw/virtio.c            | 15 ++++++++-------
>  hw/virtio.h            |  3 ++-
>  4 files changed, 20 insertions(+), 12 deletions(-)
>
> diff --git a/hw/virtio-rng.c b/hw/virtio-rng.c
> index df329f2..a73ef8e 100644
> --- a/hw/virtio-rng.c
> +++ b/hw/virtio-rng.c
> @@ -43,11 +43,11 @@ static bool is_guest_ready(VirtIORNG *vrng)
>      return false;
>  }
>  
> -static size_t get_request_size(VirtQueue *vq)
> +static size_t get_request_size(VirtQueue *vq, unsigned quota)
>  {
>      unsigned int in, out;
>  
> -    virtqueue_get_avail_bytes(vq, &in, &out);
> +    virtqueue_get_avail_bytes(vq, &in, &out, quota, 0);
>      return in;
>  }
>  
> @@ -84,12 +84,18 @@ static void chr_read(void *opaque, const void *buf, size_t size)
>  static void virtio_rng_process(VirtIORNG *vrng)
>  {
>      size_t size;
> +    unsigned quota;
>  
>      if (!is_guest_ready(vrng)) {
>          return;
>      }
>  
> -    size = get_request_size(vrng->vq);
> +    if (vrng->quota_remaining < 0) {
> +        quota = 0;
> +    } else {
> +        quota = MIN((uint64_t)vrng->quota_remaining, (uint64_t)UINT32_MAX);
> +    }
> +    size = get_request_size(vrng->vq, quota);
>      size = MIN(vrng->quota_remaining, size);
>      if (size) {
>          rng_backend_request_entropy(vrng->rng, size, chr_read, vrng);
> diff --git a/hw/virtio-serial-bus.c b/hw/virtio-serial-bus.c
> index efa8a81..155da58 100644
> --- a/hw/virtio-serial-bus.c
> +++ b/hw/virtio-serial-bus.c
> @@ -306,7 +306,7 @@ size_t virtio_serial_guest_ready(VirtIOSerialPort *port)
>      if (use_multiport(port->vser) && !port->guest_connected) {
>          return 0;
>      }
> -    virtqueue_get_avail_bytes(vq, &bytes, NULL);
> +    virtqueue_get_avail_bytes(vq, &bytes, NULL, 4096, 0);
>      return bytes;
>  }
>  
> diff --git a/hw/virtio.c b/hw/virtio.c
> index ec8b7d8..f40a8c5 100644
> --- a/hw/virtio.c
> +++ b/hw/virtio.c
> @@ -336,7 +336,8 @@ static unsigned virtqueue_next_desc(hwaddr desc_pa,
>  }
>  
>  void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
> -                               unsigned int *out_bytes)
> +                               unsigned int *out_bytes,
> +                               unsigned max_in_bytes, unsigned max_out_bytes)
>  {
>      unsigned int idx;
>      unsigned int total_bufs, in_total, out_total;
> @@ -385,6 +386,9 @@ void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
>              } else {
>                  out_total += vring_desc_len(desc_pa, i);
>              }
> +            if (in_total >= max_in_bytes && out_total >= max_out_bytes) {
> +                goto done;
> +            }
>          } while ((i = virtqueue_next_desc(desc_pa, i, max)) != max);
>  
>          if (!indirect)
> @@ -392,6 +396,7 @@ void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
>          else
>              total_bufs++;
>      }
> +done:
>      if (in_bytes) {
>          *in_bytes = in_total;
>      }
> @@ -405,12 +410,8 @@ int virtqueue_avail_bytes(VirtQueue *vq, unsigned int in_bytes,
>  {
>      unsigned int in_total, out_total;
>  
> -    virtqueue_get_avail_bytes(vq, &in_total, &out_total);
> -    if ((in_bytes && in_bytes < in_total)
> -        || (out_bytes && out_bytes < out_total)) {
> -        return 1;
> -    }
> -    return 0;
> +    virtqueue_get_avail_bytes(vq, &in_total, &out_total, in_bytes, out_bytes);
> +    return in_bytes <= in_total && out_bytes <= out_total;
>  }
>  
>  void virtqueue_map_sg(struct iovec *sg, hwaddr *addr,
> diff --git a/hw/virtio.h b/hw/virtio.h
> index df8d0f7..7c17f7b 100644
> --- a/hw/virtio.h
> +++ b/hw/virtio.h
> @@ -150,7 +150,8 @@ int virtqueue_pop(VirtQueue *vq, VirtQueueElement *elem);
>  int virtqueue_avail_bytes(VirtQueue *vq, unsigned int in_bytes,
>                            unsigned int out_bytes);
>  void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
> -                               unsigned int *out_bytes);
> +                               unsigned int *out_bytes,
> +                               unsigned max_in_bytes, unsigned max_out_bytes);
>  
>  void virtio_notify(VirtIODevice *vdev, VirtQueue *vq);
>  
> -- 
> MST