Messages in this thread | | | Date | Fri, 06 Apr 2012 13:24:42 +0800 | From | Xiao Guangrong <> | Subject | Re: [PATCH 00/13] KVM: MMU: fast page fault |
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On 04/06/2012 05:57 AM, Xiao Guangrong wrote:
>>>> >>>> What's the difference between this and >>>> >>>> >>>> if test_and_set_bit(spte.lock) >>>> return_to_guest >>>> else >>>> do checks and cmpxchg >>>> >>>> ? >>>> >>> >>> >>> test_and_set_bit is a atomic operation that is i want to avoid. >> >> Right. Can you check what the effect is (with say >> test_and_set_bit(spte, 0) in the same path). >> >> I'm not convinced it's significant. >> > > > Okay, if you prefer to test_and_set_bit, we may introduce two bits: > fast_pf and write_protect, do things like this: > > on fast page fault path: > > old_spte = *spte > > if (test_and_set_bit(spte.fast_pf)) > return_to_guest > > old_spte |= fast_pf > > if (old_spte.wp) > clear-fast-pf bit and return_to_guest > > if (!rmap.PTE_LIST_WRITE_PROTECT) > cmpxchg(spte, old_spte, old_spte +w - fast_pf) > else > clear-fast-pf bit > > on page write-protect path: > lock mmu-lock > set rmap.PTE_LIST_WRITE_PROTECT > smp_mb() > if (spte.w || spte.fast_pf) > spte -w -fast_pf + write_protect > > unlock mmu-lock > > I think it works but can not make page fault fast for unsync sp ( > e.g: two sptes point to the gfn, a page fault on one of the spte > make gfn's sp become unsync.), it is not too bad since we can cache > access bits in spte and properly make all sptes become writable on > mmu_need_write_protect path in a new patchset. >
Foolish me, i should be crazy. Sorry for my mistake. :(
Unfortunately, it can not work, we can not get a stable gfn from gpte or sp->gfns[]. For example:
beginning: Gpte = Gfn1 gfn_to_pfn(Gfn1) = Pfn Spte = Pfn Gfn1 is write-free Gfn2 is write-protected
VCPU 0 VCPU 1 VCPU 2
fault on gpte fast page fault path: set Spte.fast_pf get Gfn1 from Gpte/sp->gfns[] if (Gfn1 is writable) Pfn is swapped out: Spte = 0 Gpte is modified to Gfn2, and Pfn is realloced and remapped to Gfn2, so: Spte = Pfn
fast page fault path: set Spte.fast_pf
cmpxchg Spte+w OOPS!!! <we see Spte is not changed and happily make it writable, so gfn2 can be writable>
It seems only a unique identification can prevent this. :(
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