Re: [PATCH 2/3] virtio ring implementation

[Dor Laor <dor.laor-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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Rusty Russell wrote:
>
> These helper routines supply most of the virtqueue_ops for hypervisors
> which want to use a ring for virtio.  Unlike the previous lguest
> implementation:
>
> 1) The rings are variable sized (2^n-1 elements).
> 2) They have an unfortunate limit of 65535 bytes per sg element.
> 3) The page numbers are always 64 bit (PAE anyone?)
> 4) They no longer place used[] on a separate page, just a separate
>    cacheline.
> 5) We do a modulo on a variable.  We could be tricky if we cared.
> 6) Interrupts and notifies are suppressed using flags within the rings.
>
> Users need only implement the new_vq and free_vq hooks (KVM wants the
> guest to allocate the rings, lguest does it sanely).
>
> Signed-off-by: Rusty Russell <rusty-8n+1lVoiYb80n/F98K4Iww@public.gmane.org>
>
[snip]
>
> +irqreturn_t vring_interrupt(int irq, void *_vq)
> +{
> +       struct vring_virtqueue *vq = to_vvq(_vq);
> +
> +       pr_debug("virtqueue interrupt for %p\n", vq);
> +
> +       if (unlikely(vq->broken))
> +               return IRQ_HANDLED;
> +
> +       if (more_used(vq)) {
> +               pr_debug("virtqueue callback for %p (%p)\n",
> +                        vq, vq->vq.callback);
> +               if (!vq->vq.callback)
> +                       return IRQ_NONE;
> +               if (!vq->vq.callback(&vq->vq))
> +                       vq->vring.avail->flags |= 
> VRING_AVAIL_F_NO_INTERRUPT;
> +       } else
> +               pr_debug("virtqueue %p no more used\n", vq);
> +
> +       return IRQ_HANDLED;
> +}
> +
>
Seems like there is a problem with shared irq line, the interrupt 
handler always returns IRQ_HANDLED (except for the trivial case
were the callback is null).

It can be solved by having a host irq counter (in the shared ring) and a 
guest irq counter and return
mb(); return (host_counter!=guest_counter)? IRQ_HANDLED:IRQ_NONE;

Dor.

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