From: Rusty Russell <rusty@rustcorp.com.au>
To: Avi Kivity <avi@redhat.com>
Cc: qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org,
linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
"Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself
Date: Wed, 19 May 2010 17:09:15 +0930 [thread overview]
Message-ID: <201005191709.16401.rusty__33351.8542968251$1274254864$gmane$org@rustcorp.com.au> (raw)
In-Reply-To: <4BE9AF9A.8080005@redhat.com>
On Wed, 12 May 2010 04:57:22 am Avi Kivity wrote:
> On 05/07/2010 06:23 AM, Rusty Russell wrote:
> > On Thu, 6 May 2010 07:30:00 pm Avi Kivity wrote:
> >
> >> On 05/05/2010 11:58 PM, Michael S. Tsirkin wrote:
> >>
> >>> + /* We publish the last-seen used index at the end of the available ring.
> >>> + * It is at the end for backwards compatibility. */
> >>> + vr->last_used_idx =&(vr)->avail->ring[num];
> >>> + /* Verify that last used index does not spill over the used ring. */
> >>> + BUG_ON((void *)vr->last_used_idx +
> >>> + sizeof *vr->last_used_idx> (void *)vr->used);
> >>> }
> >>>
> >>>
> >> Shouldn't this be on its own cache line?
> >>
> > It's next to the available ring; because that's where the guest publishes
> > its data. That whole page is guest-write, host-read.
> >
> > Putting it on a cacheline by itself would be a slight pessimization; the host
> > cpu would have to get the last_used_idx cacheline and the avail descriptor
> > cacheline every time. This way, they are sometimes the same cacheline.
>
> If one peer writes the tail of the available ring, while the other reads
> last_used_idx, it's a false bounce, no?
I think we're talking about the last 2 entries of the avail ring. That means
the worst case is 1 false bounce every time around the ring. I think that's
why we're debating it instead of measuring it :)
Note that this is a exclusive->shared->exclusive bounce only, too.
Cheers,
Rusty.
next prev parent reply other threads:[~2010-05-19 7:39 UTC|newest]
Thread overview: 95+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-05-05 20:58 [PATCH RFC] virtio: put last seen used index into ring itself Michael S. Tsirkin
2010-05-05 20:58 ` [Qemu-devel] " Michael S. Tsirkin
2010-05-05 21:18 ` Dor Laor
2010-05-05 21:18 ` Dor Laor
2010-05-05 21:18 ` [Qemu-devel] " Dor Laor
2010-05-06 2:31 ` Rusty Russell
2010-05-06 2:31 ` Rusty Russell
2010-05-06 2:31 ` [Qemu-devel] " Rusty Russell
2010-05-06 6:19 ` Michael S. Tsirkin
2010-05-06 6:19 ` [Qemu-devel] " Michael S. Tsirkin
2010-05-07 3:33 ` Rusty Russell
2010-05-07 3:33 ` [Qemu-devel] " Rusty Russell
2010-05-07 3:33 ` Rusty Russell
2010-05-09 21:06 ` Michael S. Tsirkin
2010-05-09 21:06 ` Michael S. Tsirkin
2010-05-09 21:06 ` [Qemu-devel] " Michael S. Tsirkin
2010-05-06 6:19 ` Michael S. Tsirkin
2010-05-06 10:00 ` [Qemu-devel] " Avi Kivity
2010-05-06 10:00 ` Avi Kivity
2010-05-06 10:00 ` Avi Kivity
2010-05-07 3:23 ` Rusty Russell
2010-05-07 3:23 ` Rusty Russell
2010-05-07 3:23 ` Rusty Russell
2010-05-11 19:27 ` Avi Kivity
2010-05-11 19:27 ` Avi Kivity
2010-05-11 19:27 ` Avi Kivity
2010-05-11 19:52 ` Michael S. Tsirkin
2010-05-11 19:52 ` Michael S. Tsirkin
2010-05-11 19:52 ` Michael S. Tsirkin
2010-05-19 7:39 ` Rusty Russell [this message]
2010-05-19 7:39 ` Rusty Russell
2010-05-19 7:39 ` Rusty Russell
2010-05-19 8:06 ` Avi Kivity
2010-05-19 8:06 ` Avi Kivity
2010-05-19 22:33 ` Michael S. Tsirkin
2010-05-19 22:33 ` Michael S. Tsirkin
2010-05-19 22:33 ` Michael S. Tsirkin
2010-05-20 6:04 ` Avi Kivity
2010-05-20 6:04 ` Avi Kivity
2010-05-20 6:04 ` Avi Kivity
2010-05-20 5:01 ` Rusty Russell
2010-05-20 5:01 ` Rusty Russell
2010-05-20 5:01 ` Rusty Russell
2010-05-20 5:08 ` Rusty Russell
2010-05-20 5:08 ` Rusty Russell
2010-05-20 5:08 ` Rusty Russell
2010-05-23 15:31 ` Michael S. Tsirkin
2010-05-23 15:31 ` Michael S. Tsirkin
2010-05-23 15:31 ` Michael S. Tsirkin
2010-05-23 15:41 ` Avi Kivity
2010-05-23 15:41 ` Avi Kivity
2010-05-23 15:41 ` Avi Kivity
2010-05-23 15:51 ` Michael S. Tsirkin
2010-05-23 15:51 ` Michael S. Tsirkin
2010-05-23 15:51 ` Michael S. Tsirkin
2010-05-23 16:03 ` Avi Kivity
2010-05-23 16:03 ` Avi Kivity
2010-05-23 16:03 ` Avi Kivity
2010-05-23 16:30 ` Michael S. Tsirkin
2010-05-23 16:30 ` Michael S. Tsirkin
2010-05-23 16:30 ` Michael S. Tsirkin
2010-05-24 6:37 ` Avi Kivity
2010-05-24 6:37 ` Avi Kivity
2010-05-24 6:37 ` Avi Kivity
2010-05-24 8:05 ` Michael S. Tsirkin
2010-05-24 8:05 ` Michael S. Tsirkin
2010-05-24 8:05 ` Michael S. Tsirkin
2010-05-24 11:00 ` Avi Kivity
2010-05-24 11:00 ` Avi Kivity
2010-05-24 11:00 ` Avi Kivity
2010-05-23 17:28 ` Michael S. Tsirkin
2010-05-23 17:28 ` Michael S. Tsirkin
2010-05-23 17:28 ` Michael S. Tsirkin
2010-05-23 15:56 ` Michael S. Tsirkin
2010-05-23 15:56 ` Michael S. Tsirkin
2010-05-23 15:56 ` Michael S. Tsirkin
2010-05-20 7:00 ` Avi Kivity
2010-05-20 7:00 ` Avi Kivity
2010-05-20 7:00 ` Avi Kivity
2010-05-20 14:34 ` Rusty Russell
2010-05-20 14:34 ` Rusty Russell
2010-05-20 15:46 ` Avi Kivity
2010-05-20 15:46 ` Avi Kivity
2010-05-20 15:46 ` Avi Kivity
2010-05-20 14:34 ` Rusty Russell
2010-05-20 10:04 ` Michael S. Tsirkin
2010-05-20 10:04 ` Michael S. Tsirkin
2010-05-20 10:04 ` Michael S. Tsirkin
2010-05-07 3:23 ` Rusty Russell
2010-05-11 18:46 ` Ryan Harper
2010-05-11 18:46 ` Ryan Harper
2010-05-11 18:46 ` [Qemu-devel] " Ryan Harper
2010-05-11 19:48 ` Michael S. Tsirkin
2010-05-11 19:48 ` Michael S. Tsirkin
2010-05-11 19:48 ` [Qemu-devel] " Michael S. Tsirkin
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