Avi Kivity wrote: > On 06/07/2010 11:43 AM, Lai Jiangshan wrote: >> Avi Kivity wrote: >> >>> The kvm mmu synchronizes shadow ptes using the mmu lock, however the cpu >>> will happily ignore the lock when setting the accessed bit. This can >>> cause >>> the accessed bit to be lost. Luckily this only results in incorrect >>> page >>> selection for swap. >>> >>> >> Atomic operation is heavy and slow, it hurts performance. >> Incorrect page selection for swap also hurts performance. >> > > We can avoid the exchange in most cases, for example if the new spte has > the accessed bit set (already in the patch set) or if the page is > already marked as accessed, or if we see the old spte has the accessed > bit set (so no race can occur). I'll update the patches to avoid > atomics when possible.
Umm, the reason that we need atomics here is to avoid vcpu to update spte when we read A bit form it, so, perhaps we can use below way to avoid atomics completely: - set reserved bit in spte - get A bit form spte - set new spte the worst case is cause vcpu #PF here, but it doesn't matter since the old mapping is already invalid, also need a remote tlb flush later. > > I don't think atomics are that expensive, though, ~20 cycles on modern > processors? > Yes, but atomics are "LOCK" instructions, it can stop multiple cpus runing in parallel. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
