Re: [RFC 11/19] KVM: x86/mmu: Factor shadow_zero_check out of make_spte

[Sean Christopherson <seanjc@google.com>]

On Thu, Nov 18, 2021, Sean Christopherson wrote:
> On Wed, Nov 10, 2021, Ben Gardon wrote:
> > In the interest of devloping a version of make_spte that can function
> > without a vCPU pointer, factor out the shadow_zero_mask to be an
> > additional argument to the function.
> > 
> > No functional change intended.
> > 
> > 
> > Signed-off-by: Ben Gardon <bgardon@google.com>
> > ---
> >  arch/x86/kvm/mmu/spte.c | 11 +++++++----
> >  arch/x86/kvm/mmu/spte.h |  3 ++-
> >  2 files changed, 9 insertions(+), 5 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
> > index b7271daa06c5..d3b059e96c6e 100644
> > --- a/arch/x86/kvm/mmu/spte.c
> > +++ b/arch/x86/kvm/mmu/spte.c
> > @@ -93,7 +93,8 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
> >  	       struct kvm_memory_slot *slot, unsigned int pte_access,
> >  	       gfn_t gfn, kvm_pfn_t pfn, u64 old_spte, bool prefetch,
> >  	       bool can_unsync, bool host_writable, bool ad_need_write_protect,
> > -	       u64 mt_mask, u64 *new_spte)
> > +	       u64 mt_mask, struct rsvd_bits_validate *shadow_zero_check,
> 
> Ugh, so I had a big email written about how I think we should add a module param
> to control 4-level vs. 5-level for all TDP pages, but then I realized it wouldn't
> work for nested EPT because that follows the root level used by L1.  We could
> still make a global non_nested_tdp_shadow_zero_check or whatever, but then make_spte()
> would have to do some work to find the right rsvd_bits_validate, and the end result
> would likely be a mess.
> 
> One idea to avoid exploding make_spte() would be to add a backpointer to the MMU
> in kvm_mmu_page.  I don't love the idea, but I also don't love passing in rsvd_bits_validate.

Another idea.  The only difference between 5-level and 4-level is that 5-level
fills in index [4], and I'm pretty sure 4-level doesn't touch that index.  For
PAE NPT (32-bit SVM), the shadow root level will never change, so that's not an issue.

Nested NPT is the only case where anything for an EPT/NPT MMU can change, because
that follows EFER.NX.

In other words, the non-nested TDP reserved bits don't need to be recalculated
regardless of level, they can just fill in 5-level and leave it be.

E.g. something like the below.  The sp->role.direct check could be removed if we
forced EFER.NX for nested NPT.

It's a bit ugly in that we'd pass both @kvm and @vcpu, so that needs some more
thought, but at minimum it means there's no need to recalc the reserved bits.

diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index 84e64dbdd89e..05db9b89dc53 100644
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -95,10 +95,18 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
               u64 old_spte, bool prefetch, bool can_unsync,
               bool host_writable, u64 *new_spte)
 {
+       struct rsvd_bits_validate *rsvd_check;
        int level = sp->role.level;
        u64 spte = SPTE_MMU_PRESENT_MASK;
        bool wrprot = false;

+       if (vcpu) {
+               rsvd_check = vcpu->arch.mmu->shadow_zero_check;
+       } else {
+               WARN_ON_ONCE(!tdp_enabled || !sp->role.direct);
+               rsvd_check = tdp_shadow_rsvd_bits;
+       }
+
        if (sp->role.ad_disabled)
                spte |= SPTE_TDP_AD_DISABLED_MASK;
        else if (kvm_mmu_page_ad_need_write_protect(sp))
@@ -177,9 +185,9 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
        if (prefetch)
                spte = mark_spte_for_access_track(spte);

-       WARN_ONCE(is_rsvd_spte(&vcpu->arch.mmu->shadow_zero_check, spte, level),
+       WARN_ONCE(is_rsvd_spte(rsvd_check, spte, level),
                  "spte = 0x%llx, level = %d, rsvd bits = 0x%llx", spte, level,
-                 get_rsvd_bits(&vcpu->arch.mmu->shadow_zero_check, spte, level));
+                 get_rsvd_bits(rsvd_check, spte, level));

        if ((spte & PT_WRITABLE_MASK) && kvm_slot_dirty_track_enabled(slot)) {
                /* Enforced by kvm_mmu_hugepage_adjust. */