Re: [PATCH] virtio: Decrement avail idx on buffer detach

[Rusty Russell <rusty@rustcorp.com.au>]

On Wed, 16 Mar 2011 19:12:10 +0530, Amit Shah <amit.shah@redhat.com> wrote:
> When detaching a buffer from a vq, the avail.idx value should be
> decremented as well.
> 
> This was noticed by hot-unplugging a virtio console port and then
> plugging in a new one on the same number (re-using the vqs which were
> just 'disowned').  qemu reported
> 
>    'Guest moved used index from 0 to 256'
> 
> when any IO was attempted on the new port.

Yech... detach_unused_buf cannot be used on a live virtqueue; it assumes
we will reset the vq (usually by resetting the entire device).

You've partially violated that assumption by reusing the vq after
calling detach_unused_buf.  So I'm not entirely sure this is the only
bug lurking; safer would be to re-initialize the vq somehow when you
plug back in...

(Though this patch is minimal, and may be better -stable material).

Thanks,
Rusty.