On Sat, Nov 25, 2017 at 07:55:53PM +0000, Lars Ganrot wrote:
> Hi Michael and others,
>
> Hopefully my question below follows the forum format.
>
> BR,
>
> -Lars
> >
> > \subsection{In-order use of descriptors} \label{sec:Packed Virtqueues /
> > In-order
> > use of descriptors}
> >
> > Some devices always use descriptors in the same order in which they have
> > been
> > made available. These devices can offer the VIRTIO_F_IN_ORDER feature. If
> > negotiated, this knowledge allows devices to notify the use of a batch of
> > buffers
> > to the driver by only writing out a single used descriptor with the Buffer
> > ID
> > corresponding to the last descriptor in the batch.
> >
> This VIRTIO_F_IN_ORDER feature bit answers one of my earlier questions,
> however I'm curious if it couldn't also be usable in the non-ring Virtqueue?
Yes, it could.
> If the device always returns buffers in order, then couldn't the driver skip
> the step of reading the used.ring for read-only buffers (e.g. TX for net
> devices)? The used.idx tells how many buffers were returned, and since they
> are returned in the same order as the driver sent them, it knows what their
> indices are. This would then save one cache-miss in the old structure too.
True. That would be another variant to support though.
I doubt it'll outperform this one but I didn't test it
specifically. Care trying to implement it?
> And a follow-up questions would then be: if a device always returns buffers
> in order, does the v1.0 specification not require drivers to reuse
> descriptors in the same order as they are returned? I think 3.2.1.1 implies
> that at least. If so, wouldn't new descriptors always be placed back2back in
> the descriptor table (contiguous memory)?
You probably mean this:
1. Get the next free descriptor table entry, d
and you interpret "next" here as "next in ring order".
I'm not sure everyone follows this interpretation though.
E.g. Linux does:
static void detach_buf(struct vring_virtqueue *vq, unsigned int head,
void **ctx)
{
...
vq->vring.desc[i].next = cpu_to_virtio16(vq->vq.vdev, vq->free_head);
vq->free_head = head;
So descriptors are added at head of the free list. Next is interpreted
as next on this list. E.g. with a single request in flight, it looks
like a single descriptor will keep getting reused.
--
MST
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