Re: [RFC PATCH 06/15] KVM: x86/mmu: Derive page role from parent

[Sean Christopherson <seanjc@google.com>]

On Tue, Nov 30, 2021, David Matlack wrote:
> > I have a similar patch for the old MMU, but it was also replacing
> > shadow_root_level with shadow_root_role.  I'll see if I can adapt it to
> > the TDP MMU, since the shadow_root_role is obviously the same for both.
> 
> While I was writing this patch it got me wondering if we can do an
> even more general refactor and replace root_hpa and shadow_root_level
> with a pointer to the root kvm_mmu_page struct. But I didn't get a
> chance to look into it further.

For TDP MUU, yes, as root_hpa == __pa(sp->spt) in all cases.  For the legacy/full
MMU, not without additional refactoring since root_hpa doesn't point at a kvm_mmu_page
when KVM shadows a non-paging guest with PAE paging (uses pae_root), or when KVM
shadows nested NPT and the guest is using fewer paging levels that the host (uses
pml5_root or pml4_root).

	if (mmu->shadow_root_level == PT64_ROOT_5LEVEL)
		mmu->root_hpa = __pa(mmu->pml5_root);
	else if (mmu->shadow_root_level == PT64_ROOT_4LEVEL)
		mmu->root_hpa = __pa(mmu->pml4_root);
	else
		mmu->root_hpa = __pa(mmu->pae_root);

That's definitely a solvable problem, e.g. it wouldn't be a problem to burn a few
kvm_mmu_page for the special root.  The biggest issue is probably the sheer amount
of code that would need to be updated.  I do think it would be a good change, but
I think we'd want to do it in a release that isn't expected to have many other MMU
changes.

shadow_root_level can also be replaced by mmu_role.base.level.  I've never bothered
to do the replacement because there's zero memory savings and it would undoubtedly
take me some time to retrain my brain :-)