Re: [PATCH v2 17/26] KVM: x86/mmu: Pass access information to make_huge_page_split_spte()

David Matlack
2022-03-22T23:08:42Z

On Wed, Mar 16, 2022 at 1:44 AM Peter Xu <peterx@redhat.com> wrote:
>
> On Fri, Mar 11, 2022 at 12:25:19AM +0000, David Matlack wrote:
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index 85b7bc333302..541b145b2df2 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -1430,7 +1430,7 @@ static int tdp_mmu_split_huge_page(struct kvm *kvm, struct tdp_iter *iter,
> > * not been linked in yet and thus is not reachable from any other CPU.
> > */
> > for (i = 0; i < PT64_ENT_PER_PAGE; i++)
> > - sp->spt[i] = make_huge_page_split_spte(huge_spte, level, i);
> > + sp->spt[i] = make_huge_page_split_spte(huge_spte, level, i, ACC_ALL);
>
> Pure question: is it possible that huge_spte is RO while we passed in
> ACC_ALL here (which has the write bit set)?

Yes that is possible, but only if KVM the page is RO due to host-side
policies (e.g. RO memslot or RO VMA). "access" here is the
guest-allowed access permissions, similar to the pte_access parameter
to mmu_set_spte(). e.g. notice how the TDP MMU passes ACC_ALL to
make_spte().

> Would it be better if we make it a "bool exec" to be clearer?

But all that being said, the ACC_ALL stuff is confusing for exactly
the reason you pointed out so it doesn't make sense to duplicate it
further. I agree it would make more sense to pass in bool exec.

>
> --
> Peter Xu
>