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