Re: [PATCH 3/3] KVM: x86: improve reexecute_instruction

Gleb Natapov
2012-11-28T14:12:30Z

On Tue, Nov 27, 2012 at 11:30:24AM +0800, Xiao Guangrong wrote:
> On 11/27/2012 06:41 AM, Marcelo Tosatti wrote:
>
> >>
> >> - return false;
> >> +again:
> >> + page_fault_count = ACCESS_ONCE(vcpu->kvm->arch.page_fault_count);
> >> +
> >> + /*
> >> + * if emulation was due to access to shadowed page table
> >> + * and it failed try to unshadow page and re-enter the
> >> + * guest to let CPU execute the instruction.
> >> + */
> >> + kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa));
> >> + emulate = vcpu->arch.mmu.page_fault(vcpu, cr3, PFERR_WRITE_MASK, false);
> >
> > Can you explain what is the objective here?
> >
>
> Sure. :)
>
> The instruction emulation is caused by fault access on cr3. After unprotect
> the target page, we call vcpu->arch.mmu.page_fault to fix the mapping of cr3.
> if it return 1, mmu can not fix the mapping, we should report the error,
> otherwise it is good to return to guest and let it re-execute the instruction
> again.
>
> page_fault_count is used to avoid the race on other vcpus, since after we
> unprotect the target page, other cpu can enter page fault path and let the
> page be write-protected again.
>
> This way can help us to detect all the case that mmu can not be fixed.
>
Can you write this in a comment above vcpu->arch.mmu.page_fault()?

--
Gleb.