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Date: Thu, 6 May 2010 10:22:12 +0930 From: Rusty Russell <rusty@...tcorp.com.au> To: "Michael S. Tsirkin" <mst@...hat.com> Cc: netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, mingo@...e.hu, linux-mm@...ck.org, akpm@...ux-foundation.org, hpa@...or.com, gregory.haskins@...il.com, s.hetze@...ux-ag.com, Daniel Walker <dwalker@...o99.com>, Eric Dumazet <eric.dumazet@...il.com> Subject: Re: virtio: put last_used and last_avail index into ring itself. 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@...tcorp.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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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