Re: virtio: put last_used and last_avail index into ring itself.

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

On Wed, 5 May 2010 03:52:36 am Michael S. Tsirkin wrote:
> > virtio: put last_used and last_avail index into ring itself.
> > 
> > Generally, the other end of the virtio ring doesn't need to see where
> > you're up to in consuming the ring.  However, to completely understand
> > what's going on from the outside, this information must be exposed.
> > For example, if you want to save and restore a virtio_ring, but you're
> > not the consumer because the kernel is using it directly.
> > 
> > Fortunately, we have room to expand: the ring is always a whole number
> > of pages and there's hundreds of bytes of padding after the avail ring
> > and the used ring, whatever the number of descriptors (which must be a
> > power of 2).
> > 
> > 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.
> > 
> > Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> 
> I've been looking at this patch some more (more on why
> later), and I wonder: would it be better to add some
> alignment to the last used index address, so that
> if we later add more stuff at the tail, it all
> fits in a single cache line?

In theory, but not in practice.  We don't have many rings, so the
difference between 1 and 2 cache lines is not very much.

> We use a new feature bit anyway, so layout change should not be
> a problem.
> 
> Since I raised the question of caches: for used ring,
> the ring is not aligned to 64 bit, so on CPUs with 64 bit
> or larger cache lines, used entries will often cross
> cache line boundaries. Am I right and might it
> have been better to align ring entries to cache line boundaries?
> 
> What do you think?

I think everyone is settled on 128 byte cache lines for the forseeable
future, so it's not really an issue.

Cheers,
Rusty.

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