Re: [Qemu-devel] [PATCH] Tap: fix vcpu long time io blocking on tap

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Jul 11, 2014 at 01:05:30AM +0000, Wangkai (Kevin,C) wrote:
> When used a tap as net driver for vm, if too many packets was delivered to the 
> guest os via tap interface, the guest os will be blocked on io events for a long
> time, while tap driver was busying process packets.

Please tweak this description to explain clearly that the tap_send()
loop can execute for a very long time if more packets are received by
the host during tap_send().

It seems that the guest does not require the QEMU global mutex often
during virtio-net receive, otherwise the rx virtqueue would fill up and
tap_send() would return.

> diff --git a/net/tap.c b/net/tap.c
> index a40f7f0..df9a0eb 100644
> --- a/net/tap.c
> +++ b/net/tap.c
> @@ -189,6 +189,7 @@ static void tap_send(void *opaque)
>  {
>      TAPState *s = opaque;
>      int size;
> +    int pkt = 0;
>  
>      while (qemu_can_send_packet(&s->nc)) {
>          uint8_t *buf = s->buf;
> @@ -210,6 +211,11 @@ static void tap_send(void *opaque)
>          } else if (size < 0) {
>              break;
>          }
> +
> +        /* limit io block time slice for 50 packets */
> +        pkt++;
> +        if (pkt >= 50)
> +            break;

Please use scripts/checkpatch.pl to check coding style.  QEMU always
uses curlies around if statement bodies, even when they are only one
line.

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