On (Thu) 17 Mar 2011 [15:26:28], Rusty Russell wrote:
> On Wed, 16 Mar 2011 19:12:10 +0530, Amit Shah <[email protected]> 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...
Right; and then that will need host changes too (re-init the vqs on
the host side), which then gets us into compat problems...
Amit
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