Re: [PATCH 03/11] KVM: x86: retry non-page-table writing instruction

Marcelo Tosatti
2011-08-22T20:43:03Z

On Tue, Aug 23, 2011 at 04:21:05AM +0800, Xiao Guangrong wrote:
> On 08/23/2011 03:59 AM, Marcelo Tosatti wrote:
>
> >> + if (!vcpu->arch.mmu.direct_map && !mmu_is_nested(vcpu))
> >> + gpa = kvm_mmu_gva_to_gpa_write(vcpu, cr2, NULL);
> >
> > Why write?
> >
>
> Since the fault is caused by page table written, and the 'gpa' can
> be written after instruction is retied.
>
> >> + kvm_mmu_unprotect_page(vcpu->kvm, gpa >> PAGE_SHIFT);
> >> +
> >> + /*
> >> + * The shadow pages have been zapped, then we call the page
> >> + * fault path to change the mapping to writable.
> >> + */
> >> + vcpu->arch.mmu.page_fault(vcpu, cr2, PFERR_WRITE_MASK, true);
> >
> > I don't see why is this necessary. Just allowing the instruction to
> > proceed should be enough?
> >
>
> It used to avoid later VM-exit, since we will retry the instruction
> but the mapped is still read-only. So we can it to let the mapping become
> writable to avoid page fault again.

Its not like this case is performance sensitive. Usually optimizing
things without the need for it leads to bad results. So please drop
this.