Re: [PATCH] virtio-spec: 64 bit features, used/avail event

[Avi Kivity <avi@redhat.com>]

On 05/02/2011 04:17 PM, Michael S. Tsirkin wrote:
> I'm working on a patchset that modified the notificatin
> hand-off in virtio to be basically like Xen:
> each side published an index, the other side only triggers
> an event when it crosses that index value
> (Xen event indexes start at 1, ours start at 0 for
> backward-compatiblity, but that's minor).
>
> Especially for testing, it is very convenient to have
> separate feature bits for this change in used and available
> ring; since we've run out of bits in the 32 bit field,
> I added another 32 bit and bit 31 enables that.
>

> -30 Feature bits reserved for extensions to the queue mechanism
> +
> +\change_inserted 0 1304329326
> +4
> +\change_deleted 0 1304329325
> +3
> +\change_unchanged
> +0 Feature bits reserved for extensions to the queue mechanism

That would be 24 to 30, 32 to 40.

> +\change_inserted 0 1304329351
> +
> +\end_layout
> +
> +\begin_layout Description
> +
> +\change_inserted 0 1304329398
> +41
> +\begin_inset space ~
> +\end_inset
> +
> +to
> +\begin_inset space ~
> +\end_inset
> +
> +63 Feature bits reserved for future extensions
>   \end_layout
>
>
> @@ -1891,7 +2118,38 @@ flags
>
>    field is currently 0 or 1: 1 indicating that we do not need an interrupt
>    when the device consumes a descriptor from the available ring.
> - This interrupt suppression is merely an optimization; it may not suppress
> +
> +\change_inserted 0 1304331587
> +Alternatively, we can ask the device to delay interrupts until an entry

we can ask -> the guest may ask

> + with an index specified by the
> +\begin_inset Quotes eld
> +\end_inset
> +
> +used_event
> +\begin_inset Quotes erd
> +\end_inset
> +
> + field is written in the used ring (equivalently, until the
> +\emph on
> +idx
> +\emph default
> + field in the used ring will reach the value
> +\emph on
> +used_event + 1
> +\emph default
> +).
> + The method employed by the device is controlled by the VIRTIO_RING_F_USED_EVENT
> +_IDX feature bit (see
> +\begin_inset CommandInset ref
> +LatexCommand ref
> +reference "cha:Reserved-Feature-Bits"
> +
> +\end_inset
> +
> +).
> +
> +\change_unchanged
> +This interrupt suppression is merely an optimization; it may not suppress
>    interrupts entirely.
>   \end_layout

This section is strangely worded, from the guest point of view, which is 
strange for the device spec.  It's better to say things explicitly.  
However, this issue is not introduced by the patch.

>
> @@ -1940,6 +2198,17 @@ struct vring_avail {
>   \begin_layout Plain Layout
>
>      u16 ring[qsz]; /* qsz is the Queue Size field read from device */
> +\change_inserted 0 1304329945
> +
> +\end_layout
> +
> +\begin_layout Plain Layout
> +
> +\change_inserted 0 1304329957
> +
> +   u16 used_event;
> +\change_unchanged
> +
>   \end_layout
>
>   \begin_layout Plain Layout
> @@ -1963,8 +2232,63 @@ The used ring is where the device returns buffers once it is done with them.
>   \emph on
>   available
>   \emph default
> - ring (the flag is kept here because this is the only part of the virtqueue
> - written by the device).
> + ring
> +\change_inserted 0 1304331253
> +.
> + Alternatively, the device can hint that no notification is necessary until

the guest can request the device to hint


-- 
error compiling committee.c: too many arguments to function