Messages in this thread | | | Date | Fri, 06 Apr 2012 02:25:33 +0800 | From | Xiao Guangrong <> | Subject | Re: [PATCH 09/13] KVM: MMU: get expected spte out of mmu-lock |
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On 04/01/2012 11:53 PM, Avi Kivity wrote:
> On 03/29/2012 11:25 AM, Xiao Guangrong wrote: >> It depends on PTE_LIST_WRITE_PROTECT bit in rmap which let us quickly know >> whether the page is writable out of mmu-lock >> >> Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com> >> --- >> arch/x86/kvm/mmu.c | 17 +++++++++++++---- >> arch/x86/kvm/paging_tmpl.h | 2 +- >> 2 files changed, 14 insertions(+), 5 deletions(-) >> >> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c >> index 3887a07..c029185 100644 >> --- a/arch/x86/kvm/mmu.c >> +++ b/arch/x86/kvm/mmu.c >> @@ -1148,6 +1148,12 @@ static int rmap_write_protect(struct kvm *kvm, u64 gfn) >> >> *rmapp |= PTE_LIST_WRITE_PROTECT; >> >> + /* >> + * Setting PTE_LIST_WRITE_PROTECT bit before doing page >> + * write-protect. >> + */ >> + smp_mb(); >> + > > wmb only needed. >
We should ensure setting this bit before reading spte, it cooperates with fast page fault path to avoid this case:
On fast page fault path: On rmap_write_protect path: read spte: old_spte = *spte (reading spte is reordered to the front of setting PTE_LIST_WRITE_PROTECT bit) set spte.identification smp_mb if (!rmap.PTE_LIST_WRITE_PROTECT) set rmap.PTE_LIST_WRITE_PROTECT cmpxchg(sptep, spte, spte | WRITABLE) see old_spte.identification is not set, so it does not write-protect this page OOPS!!!
> Would it be better to store this bit in all the sptes instead? We're > touching them in any case. More work to clear them, but > un-write-protecting a page is beneficial anyway as it can save a fault. >
There are two reasons: - if we set this bit in rmap, we can do the quickly check to see the page is writble before doing shadow page walking.
- since a full barrier is needed, we should use smp_mb for every spte like this:
while ((spte = rmap_next(rmapp, spte))) { read spte smp_mb write-protect spte }
smp_mb is called in the loop, i think it is not good, yes?
If you just want to save the fault, we can let all spte to be writeable in mmu_need_write_protect, but we should cache gpte access bits into spte firstly. It should be another patchset i think. :)
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