Re: [PATCH 3/3] KVM: arm64: Failback on unsupported huge page sizes

[Marc Zyngier]

On 2020-10-25 23:04, Gavin Shan wrote:
> Hi Marc,
>
> On 10/25/20 9:48 PM, Marc Zyngier wrote:
> > On Sun, 25 Oct 2020 01:27:39 +0100,
> > Gavin Shan <gshan@xxxxxxxxxx> wrote:
> > > The huge page could be mapped through multiple contiguous PMDs or 
> > > PTEs.
> > > The corresponding huge page sizes aren't supported by the page table
> > > walker currently.
> > >
> > > This fails the unsupported huge page sizes to the near one. 
> > > Otherwise,
> > > the guest can't boot successfully: CONT_PMD_SHIFT and CONT_PTE_SHIFT
> > > fail back to PMD_SHIFT and PAGE_SHIFT separately.
> > >
> > > Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
> > > ---
> > >   arch/arm64/kvm/mmu.c | 8 ++++++++
> > >   1 file changed, 8 insertions(+)
> > >
> > > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > > index 0f51585adc04..81cbdc368246 100644
> > > --- a/arch/arm64/kvm/mmu.c
> > > +++ b/arch/arm64/kvm/mmu.c
> > > @@ -793,12 +793,20 @@ static int user_mem_abort(struct kvm_vcpu 
> > > *vcpu, phys_addr_t fault_ipa,
> > >   		vma_shift = PMD_SHIFT;
> > >   #endif
> > >   +	if (vma_shift == CONT_PMD_SHIFT)
> > > +		vma_shift = PMD_SHIFT;
> > > +
> > >   	if (vma_shift == PMD_SHIFT &&
> > >   	    !fault_supports_stage2_huge_mapping(memslot, hva, PMD_SIZE)) {
> > >   		force_pte = true;
> > >   		vma_shift = PAGE_SHIFT;
> > >   	}
> > >   +	if (vma_shift == CONT_PTE_SHIFT) {
> > > +		force_pte = true;
> > > +		vma_shift = PAGE_SHIFT;
> > > +	}
> > > +
> > >   	vma_pagesize = 1UL << vma_shift;
> > >   	if (vma_pagesize == PMD_SIZE || vma_pagesize == PUD_SIZE)
> > >   		fault_ipa &= ~(vma_pagesize - 1);
> >
> > Yup, nice catch. However, I think we should take this opportunity to
> > rationalise the logic here, and catch future discrepancies (should
> > someone add contiguous PUD or something similarly silly). How about
> > something like this (untested):
>
> Yeah, I started the work to support contiguous PMDs/PTEs, but I'm not
> sure when I can post the patches for review as my time becomes a bit
> fragmented recently. At least, I need focus on "async page fault" in
> the coming weeks :)
>
> Thanks for the suggested code and it worked for me. I'll post v2 to
> integrate them. However, I would like to drop PATCH[1] and PATCH[2]
> as I really don't have strong reasons to have them.

Yes, please drop these patches, and focus on the actual bug fix.

Thanks,

        M.
--
Jazz is not dead. It just smells funny...