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Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring

From: Avi Kivity
Subject: Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself
Date: Tue, 11 May 2010 22:27:22 +0300
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100330 Fedora/3.0.4-1.fc12 Thunderbird/3.0.4

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?

Having things on the same cacheline is only worthwhile if they are accessed at the same time.

Do not meddle in the internals of kernels, for they are subtle and quick to 

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