[RFC PATCH 20/37] KVM: x86/mmu: Abstract away computing the max mapping level

[Sean Christopherson]

On Mon, Dec 12, 2022, David Matlack wrote:
> On Mon, Dec 12, 2022 at 11:32 AM Ben Gardon <bgardon at google.com> wrote:
> >
> > On Thu, Dec 8, 2022 at 11:39 AM David Matlack <dmatlack at google.com> wrote:
> > >
> > > Abstract away kvm_mmu_max_mapping_level(), which is an x86-specific
> > > function for computing the max level that a given GFN can be mapped in
> > > KVM's page tables. This will be used in a future commit to enable moving
> > > the TDP MMU to common code.
> > >
> > > Provide a default implementation for non-x86 architectures that just
> > > returns the max level. This will result in more zapping than necessary
> > > when disabling dirty logging (i.e. less than optimal performance) but no
> > > correctness issues.
> >
> > Apologies if you already implemented it in a later patch in this
> > series, but would it not at least be possible to port
> > host_pfn_mapping_level to common code and check that?
> > I'm assuming, though I could be wrong, that all archs map GFNs with at
> > most a host page table granularity mapping.
> > I suppose that doesn't strictly need to be included in this series,
> > but it would be worth addressing in the commit description.
> 
> It's not implemented later in this series, but I agree it's something
> we should do. In fact, it's worth doing regardless of this series as a
> way to share more code across architectures (e.g. KVM/ARM has it's own
> version in arch/arm64/kvm/mmu.c:get_user_mapping_size()).

Ya, ARM converted to walking the host user page tables largely in response to
x86's conversion.  After x86 switched, ARM was left holding the bag that was
PageTransCompoundMap().

On a related topic, I'm guessing all the comments in transparent_hugepage_adjust()
about the code working _only_ for THP are stale.  Unless ARM support for HugeTLB
works differently, walking host user page tables should Just Work for all hugepage
types.