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SubjectRe: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself
On Tue, May 11, 2010 at 10:27:22PM +0300, 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?
>
> Having things on the same cacheline is only worthwhile if they are
> accessed at the same time.

Yes, this is what I was trying to say.
avail flags and used index *are* accessed at the same time, so
there could be an advantage to sharing a cache line there.

All this should be kept in mind if we ever do
VIRTIO_RING_F_NEW_LAYOUT.

--
MST


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