Re: [PATCH 08/13] KVM: MMU: fask check whether page is writable

Marcelo Tosatti
2012-04-12T23:39:52Z

On Fri, Apr 06, 2012 at 01:54:52AM +0800, Xiao Guangrong wrote:
> Hi Avi,
>
> Thanks very much for your review!
>
> Sorry for the delay reply since i was on vacation.
>
> On 04/01/2012 11:52 PM, Avi Kivity wrote:
>
> > On 03/29/2012 11:25 AM, Xiao Guangrong wrote:
> >> Using PTE_LIST_WRITE_PROTECT bit in rmap store the write-protect status to
> >> avoid unnecessary shadow page walking
> >>
> >> Also if no shadow page is indirect, the page is write-free
> >>
> >>
> >> @@ -2262,6 +2291,9 @@ static int mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn,
> >> }
> >> if (need_unsync)
> >> kvm_unsync_pages(vcpu, gfn);
> >> +
> >> + *rmap &= ~PTE_LIST_WRITE_PROTECT;
> >> +
> >>
> >
> > So what are the rules for PTE_LIST_WRITE_PROTECT? Is is a cache for the
> > mmu_need_write_protect?
> >
> > I'd like to understand it, I guess it can be set while write protection
> > is unneeded, and cleared on the next check?
> >
>
>
> Yes, it is used as a cache for mmu_need_write_protect.
>
> When the gfn is protected by sync sp or read-only host page we set this bit,
> and it is be cleared when the sp become unsync or host page becomes writable.

Wouldnt dropping support for shadow entirely make it much simpler?

The idea to handle RO->RW without mmu_lock is very neat, but the
complexity with shadow is horrible.