On Thu, May 06, 2010 at 10:22:12AM +0930, Rusty Russell wrote: > 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@xxxxxxxxxxxxxxx> > > > > 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. Fair enough. > > 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. You mean with 64 bit descriptors we will be bouncing a cache line between host and guest, anyway? -- MST -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
