[Qemu-devel] Re: [PATCHv2-RFC 0/2] virtio: put last seen used index into ring itself

[Jes Sorensen]

On 05/26/10 21:50, Michael S. Tsirkin wrote:
> Here's a rewrite of the original patch with a new layout.
> I haven't tested it yet so no idea how this performs, but
> I think this addresses the cache bounce issue raised by Avi.
> Posting for early flames/comments.
> 
> Generally, the Host end of the virtio ring doesn't need to see where
> Guest is up to in consuming the ring.  However, to completely understand
> what's going on from the outside, this information must be exposed.
> For example, host can reduce the number of interrupts by detecting
> that the guest is currently handling previous buffers.
> 
> We add a feature bit so the guest can tell the host that it's writing
> out the current value there, if it wants to use that.
> 
> This differs from original approach in that the used index
> is put after avail index (they are typically written out together).
> To avoid cache bounces on descriptor access,
> and make future extensions easier, we put the ring itself at start of
> page, and move the control after it.

Hi Michael,

It looks pretty good to me, however one thing I have been thinking of
while reading through it:

Rather than storing a pointer within the ring struct, pointing into a
position within the same struct. How about storing a byte offset instead
and using a cast to get to the pointer position? That would avoid the
pointer dereference, which is less effective cache wise and harder for
the CPU to predict.

Not sure whether it really matters performance wise, just a thought.

Cheers,
Jes