Re: [PATCH 2/5] KVM: arm64: Avoid mapping size adjustment on permission fault

[Marc Zyngier <maz@kernel.org>]

On Fri, 23 Jul 2021 16:55:39 +0100,
Alexandru Elisei <alexandru.elisei@arm.com> wrote:
> 
> Hi Marc,
> 
> On 7/17/21 10:55 AM, Marc Zyngier wrote:
> > Since we only support PMD-sized mappings for THP, getting
> > a permission fault on a level that results in a mapping
> > being larger than PAGE_SIZE is a sure indication that we have
> > already upgraded our mapping to a PMD.
> >
> > In this case, there is no need to try and parse userspace page
> > tables, as the fault information already tells us everything.
> >
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > ---
> >  arch/arm64/kvm/mmu.c | 11 ++++++++---
> >  1 file changed, 8 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index db6314b93e99..c036a480ca27 100644
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -1088,9 +1088,14 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> >  	 * If we are not forced to use page mapping, check if we are
> >  	 * backed by a THP and thus use block mapping if possible.
> >  	 */
> > -	if (vma_pagesize == PAGE_SIZE && !(force_pte || device))
> > -		vma_pagesize = transparent_hugepage_adjust(kvm, memslot, hva,
> > -							   &pfn, &fault_ipa);
> > +	if (vma_pagesize == PAGE_SIZE && !force_pte) {
> 
> Looks like now it's possible to call transparent_hugepage_adjust()
> for devices (if fault_status != FSC_PERM). Commit 2aa53d68cee6
> ("KVM: arm64: Try stage2 block mapping for host device MMIO") makes
> a good case for the !device check. Was the check removed
> unintentionally?

That's what stupid bugs are made of. I must have resolved a rebase
conflict the wrong way, and lost this crucial bit. Thanks for spotting
this! Now fixed.

> 
> > +		if (fault_status == FSC_PERM && fault_granule > PAGE_SIZE)
> > +			vma_pagesize = fault_granule;
> > +		else
> > +			vma_pagesize = transparent_hugepage_adjust(kvm, memslot,
> > +								   hva, &pfn,
> > +								   &fault_ipa);
> > +	}
> 
> This change makes sense to me - we can only get stage 2 permission
> faults on a leaf entry since stage 2 tables don't have the
> APTable/XNTable/PXNTable bits. The biggest block mapping size that
> we support at stage 2 is PMD size (from
> transparent_hugepage_adjust()), therefore if fault_granule is larger
> than PAGE_SIZE, then it must be PMD_SIZE.

Yup, exactly.

Thanks again,

	M.

-- 
Without deviation from the norm, progress is not possible.