Re: [Qemu-devel] [PATCH v4 04/13] virtio: poll virtqueues for new buffers

[Fam Zheng <famz@redhat.com>]

On Thu, 12/01 19:26, Stefan Hajnoczi wrote:
> Add an AioContext poll handler to detect new virtqueue buffers without
> waiting for a guest->host notification.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  hw/virtio/virtio.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 6f8ca25..3870411 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -2047,13 +2047,27 @@ static void virtio_queue_host_notifier_aio_read(EventNotifier *n)
>      }
>  }
>  
> +static bool virtio_queue_host_notifier_aio_poll(void *opaque)
> +{
> +    EventNotifier *n = opaque;
> +    VirtQueue *vq = container_of(n, VirtQueue, host_notifier);
> +
> +    if (virtio_queue_empty(vq)) {

I notice this call path gets very hot in the profile:

#0  address_space_translate_internal (d=0x7f853c22e970, addr=2140788738, 
    xlat=xlat@entry=0x7f8549db59e8, plen=plen@entry=0x7f8549db5a68, 
    resolve_subpage=resolve_subpage@entry=true) at /root/qemu-nvme/exec.c:419
#1  0x00007f854d3e8449 in address_space_translate (as=<optimized out>, addr=2140788738, 
    xlat=xlat@entry=0x7f8549db5a70, plen=plen@entry=0x7f8549db5a68, 
    is_write=is_write@entry=false) at /root/qemu-nvme/exec.c:462
#2  0x00007f854d3ee2fd in address_space_lduw_internal (endian=DEVICE_LITTLE_ENDIAN, 
    result=0x0, attrs=..., addr=<optimized out>, as=<optimized out>)
    at /root/qemu-nvme/exec.c:3299
#3  address_space_lduw_le (result=0x0, attrs=..., addr=<optimized out>, as=<optimized out>)
    at /root/qemu-nvme/exec.c:3351
#4  lduw_le_phys (as=<optimized out>, addr=<optimized out>) at /root/qemu-nvme/exec.c:3369
#5  0x00007f854d475978 in virtio_lduw_phys (vdev=<optimized out>, pa=<optimized out>)
    at /root/qemu-nvme/include/hw/virtio/virtio-access.h:46
#6  vring_avail_idx (vq=<optimized out>, vq=<optimized out>)
    at /root/qemu-nvme/hw/virtio/virtio.c:143
#7  virtio_queue_empty (vq=vq@entry=0x7f854fd0c9e0) at /root/qemu-nvme/hw/virtio/virtio.c:246
#8  0x00007f854d475d20 in virtio_queue_empty (vq=0x7f854fd0c9e0)
    at /root/qemu-nvme/hw/virtio/virtio.c:2074
#9  virtio_queue_host_notifier_aio_poll (opaque=0x7f854fd0ca48)
    at /root/qemu-nvme/hw/virtio/virtio.c:2068
#10 0x00007f854d69116e in run_poll_handlers_once (ctx=<optimized out>) at aio-posix.c:493
#11 0x00007f854d691b08 in run_poll_handlers (max_ns=<optimized out>, ctx=0x7f854e908850)
    at aio-posix.c:530
#12 try_poll_mode (blocking=true, ctx=0x7f854e908850) at aio-posix.c:558
#13 aio_poll (ctx=0x7f854e908850, blocking=blocking@entry=true) at aio-posix.c:601
#14 0x00007f854d4ff12e in iothread_run (opaque=0x7f854e9082f0) at iothread.c:53
#15 0x00007f854c249dc5 in start_thread () from /lib64/libpthread.so.0
#16 0x00007f854acd973d in clone () from /lib64/libc.so.6

Is it too costly to do an address_space_translate per poll?

Fam

> +        return false;
> +    }
> +
> +    virtio_queue_notify_aio_vq(vq);
> +    return true;
> +}
> +
>  void virtio_queue_aio_set_host_notifier_handler(VirtQueue *vq, AioContext *ctx,
>                                                  VirtIOHandleOutput handle_output)
>  {
>      if (handle_output) {
>          vq->handle_aio_output = handle_output;
>          aio_set_event_notifier(ctx, &vq->host_notifier, true,
> -                               virtio_queue_host_notifier_aio_read, NULL);
> +                               virtio_queue_host_notifier_aio_read,
> +                               virtio_queue_host_notifier_aio_poll);
>      } else {
>          aio_set_event_notifier(ctx, &vq->host_notifier, true, NULL, NULL);
>          /* Test and clear notifier before after disabling event,
> -- 
> 2.9.3
>