Re: [PATCH 1/5] KVM: arm64: Walk userspace page tables to compute the THP mapping size

[Marc Zyngier <maz@kernel.org>]

On Mon, 19 Jul 2021 07:31:30 +0100,
Paolo Bonzini <pbonzini@redhat.com> wrote:
> 
> On 17/07/21 11:55, Marc Zyngier wrote:
> > We currently rely on the kvm_is_transparent_hugepage() helper to
> > discover whether a given page has the potential to be mapped as
> > a block mapping.
> > 
> > However, this API doesn't really give un everything we want:
> > - we don't get the size: this is not crucial today as we only
> >    support PMD-sized THPs, but we'd like to have larger sizes
> >    in the future
> > - we're the only user left of the API, and there is a will
> >    to remove it altogether
> > 
> > To address the above, implement a simple walker using the existing
> > page table infrastructure, and plumb it into transparent_hugepage_adjust().
> > No new page sizes are supported in the process.
> > 
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> 
> If it's okay for you to reuse the KVM page walker that's fine of
> course, but the arch/x86/mm functions lookup_address_in_{mm,pgd} are
> mostly machine-independent and it may make sense to move them to mm/.
>
> That would also allow reusing the x86 function host_pfn_mapping_level.

That could work to some extent, but the way the x86 code equates level
to mapping size is a bit at odds with the multiple page sizes that
arm64 deals with.

We're also trying to move away from the whole P*D abstraction, because
this isn't something we want to deal with in the self-contained EL2
object.

Thanks,

	M.

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