netdev - Re: [PATCH RFC v2 1/4] virtio: fix up virtio_disable

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Date:   Tue, 13 Apr 2021 20:24:48 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Jason Wang <jasowang@...hat.com>, Wei Wang <weiwan@...gle.com>,
        David Miller <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH RFC v2 1/4] virtio: fix up virtio_disable_cb

> > >
> > >
> > > but even yours is also fixed I think.
> > >
> > > The common point is that a single spurious interrupt is not a problem.
> > > The problem only exists if there are tons of spurious interrupts with no
> > > real ones. For this to trigger, we keep polling the ring and while we do
> > > device keeps firing interrupts. So just disable interrupts while we
> > > poll.
> >
> > But the main change in this patch is to turn some virtqueue_disable_cb
> > calls into no-ops.
>
> Well this was not the design. This is the main change:
>
>
> @@ -739,7 +742,10 @@ static void virtqueue_disable_cb_split(struct virtqueue *_vq)
>
>         if (!(vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
>                 vq->split.avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
> -               if (!vq->event)
> +               if (vq->event)
> +                       /* TODO: this is a hack. Figure out a cleaner value to write. */
> +                       vring_used_event(&vq->split.vring) = 0x0;
> +               else
>                         vq->split.vring.avail->flags =
>                                 cpu_to_virtio16(_vq->vdev,
>                                                 vq->split.avail_flags_shadow);
>
>
> IIUC previously when event index was enabled (vq->event) virtqueue_disable_cb_split
> was a nop. Now it sets index to 0x0 (which is a hack, but good enough
> for testing I think).

So now tx interrupts will really be suppressed even in event-idx mode.

And what is the purpose of suppressing this operation if
event_triggered, i.e., after an interrupt occurred? You mention " if
using event index with a packed ring, and if being called from a
callback, we actually do disable interrupts which is unnecessary." Can
you elaborate? Also, even if unnecessary, does it matter? The
operation itself seems fairly cheap.

These should probably be two separate patches.

There is also a third case, split ring without event index. That
behaves more like packed ring, I suppose.


> > I don't understand how that helps reduce spurious
> > interrupts, as if anything, it keeps interrupts enabled for longer.