Messages in this thread | | | Date | Mon, 26 Apr 2010 09:07:54 +0300 | From | Avi Kivity <> | Subject | Re: [PATCH v2 6/10] KVM MMU: don't write-protect if have new mapping to unsync page |
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On 04/26/2010 06:58 AM, Xiao Guangrong wrote: > > Avi Kivity wrote: > >> On 04/25/2010 10:00 AM, Xiao Guangrong wrote: >> >>> Two cases maybe happen in kvm_mmu_get_page() function: >>> >>> - one case is, the goal sp is already in cache, if the sp is unsync, >>> we only need update it to assure this mapping is valid, but not >>> mark it sync and not write-protect sp->gfn since it not broke unsync >>> rule(one shadow page for a gfn) >>> >>> - another case is, the goal sp not existed, we need create a new sp >>> for gfn, i.e, gfn (may)has another shadow page, to keep unsync rule, >>> we should sync(mark sync and write-protect) gfn's unsync shadow page. >>> After enabling multiple unsync shadows, we sync those shadow pages >>> only when the new sp not allow to become unsync(also for the unsyc >>> rule, the new rule is: allow all pte page become unsync) >>> >>> >> Another interesting case is to create new shadow pages in the unsync >> state. That can help when the guest starts a short lived process: we >> can avoid write protecting its pagetables completely. Even if we do >> sync them, we can sync them in a batch instead of one by one, saving IPIs. >> > IPI is needed when rmap_write_protect() changes mappings form writable to read-only, > so while we sync all gfn's unsync page, only one IPI is needed. >
I meant, we can write protect all pages, then use one IPI to drop the tlbs for all of them.
> And, another problem is we call ramp_write_protect()/flush-local-tlb many times when sync gfn's > unsync page, the same problem is in mmu_sync_children() function, could you allow me to improve > it after this patchset? :-) >
Of course, this is more than enough to chew on. Just suggesting an idea...
-- Do not meddle in the internals of kernels, for they are subtle and quick to panic.
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