From: "Michael S. Tsirkin" <mst@redhat.com> To: Rusty Russell <rusty@rustcorp.com.au> Cc: netdev@vger.kernel.org, virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, mingo@elte.hu, linux-mm@kvack.org, akpm@linux-foundation.org, hpa@zytor.com, gregory.haskins@gmail.com, s.hetze@linux-ag.com, Daniel Walker <dwalker@fifo99.com>, Eric Dumazet <eric.dumazet@gmail.com> Subject: Re: virtio: put last_used and last_avail index into ring itself. Date: Thu, 6 May 2010 09:27:55 +0300 [thread overview] Message-ID: <20100506062755.GC8363@redhat.com> (raw) In-Reply-To: <201005061022.13815.rusty@rustcorp.com.au> 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@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. 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
WARNING: multiple messages have this Message-ID (diff)
From: "Michael S. Tsirkin" <mst@redhat.com> To: Rusty Russell <rusty@rustcorp.com.au> Cc: netdev@vger.kernel.org, virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, mingo@elte.hu, linux-mm@kvack.org, akpm@linux-foundation.org, hpa@zytor.com, gregory.haskins@gmail.com, s.hetze@linux-ag.com, Daniel Walker <dwalker@fifo99.com>, Eric Dumazet <eric.dumazet@gmail.com> Subject: Re: virtio: put last_used and last_avail index into ring itself. Date: Thu, 6 May 2010 09:27:55 +0300 [thread overview] Message-ID: <20100506062755.GC8363@redhat.com> (raw) In-Reply-To: <201005061022.13815.rusty@rustcorp.com.au> 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@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. 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, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2010-05-06 6:33 UTC|newest] Thread overview: 51+ messages / expand[flat|nested] mbox.gz Atom feed top [not found] <cover.1257349249.git.mst@redhat.com> 2009-11-04 15:55 ` [PATCHv8 1/3] tun: export underlying socket Michael S. Tsirkin 2009-11-04 15:55 ` Michael S. Tsirkin 2009-11-04 15:55 ` Michael S. Tsirkin 2009-11-04 15:55 ` Michael S. Tsirkin 2009-11-04 15:55 ` [PATCHv8 2/3] mm: export use_mm/unuse_mm to modules Michael S. Tsirkin 2009-11-04 15:55 ` Michael S. Tsirkin 2009-11-04 15:55 ` Michael S. Tsirkin 2009-11-04 15:55 ` Michael S. Tsirkin 2009-11-04 15:57 ` [PATCHv8 3/3] vhost_net: a kernel-level virtio server Michael S. Tsirkin 2009-11-04 15:57 ` Michael S. Tsirkin 2009-11-04 15:57 ` Michael S. Tsirkin 2009-11-04 15:57 ` Michael S. Tsirkin 2009-11-06 4:59 ` Rusty Russell 2009-11-06 4:59 ` Rusty Russell 2009-11-06 4:59 ` Rusty Russell 2009-11-08 11:35 ` Michael S. Tsirkin 2009-11-08 11:35 ` Michael S. Tsirkin 2009-11-09 6:17 ` Rusty Russell 2009-11-09 6:17 ` Rusty Russell 2009-11-09 6:17 ` Rusty Russell 2009-11-09 7:10 ` Michael S. Tsirkin 2009-11-09 7:10 ` Michael S. Tsirkin 2009-11-10 1:08 ` Rusty Russell 2009-11-10 1:08 ` Rusty Russell 2009-11-10 1:08 ` Rusty Russell 2009-11-09 7:10 ` Michael S. Tsirkin 2009-11-09 7:20 ` Michael S. Tsirkin 2009-11-09 7:20 ` Michael S. Tsirkin 2009-11-09 7:20 ` Michael S. Tsirkin 2009-11-09 11:55 ` Michael S. Tsirkin 2009-11-09 11:55 ` Michael S. Tsirkin 2009-11-09 11:55 ` Michael S. Tsirkin 2010-05-04 18:22 ` virtio: put last_used and last_avail index into ring itself Michael S. Tsirkin 2010-05-04 18:22 ` Michael S. Tsirkin 2010-05-06 0:52 ` Rusty Russell 2010-05-06 0:52 ` Rusty Russell 2010-05-06 0:52 ` Rusty Russell 2010-05-06 6:27 ` Michael S. Tsirkin 2010-05-06 6:27 ` Michael S. Tsirkin [this message] 2010-05-06 6:27 ` Michael S. Tsirkin 2010-05-07 3:05 ` Rusty Russell 2010-05-07 3:05 ` Rusty Russell 2010-05-07 3:05 ` Rusty Russell 2010-05-09 8:57 ` Michael S. Tsirkin 2010-05-09 8:57 ` Michael S. Tsirkin 2010-05-09 8:57 ` Michael S. Tsirkin 2010-05-10 3:11 ` Rusty Russell 2010-05-10 3:11 ` Rusty Russell 2010-05-10 3:11 ` Rusty Russell 2010-05-04 18:22 ` Michael S. Tsirkin 2009-11-08 11:35 ` [PATCHv8 3/3] vhost_net: a kernel-level virtio server Michael S. Tsirkin
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