[PATCH RFC] virtio: put last seen used index into ring itself
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From: Avi Kivity on 11 May 2010 15:30 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 panic. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |