KVM: x86/mmu: Don't WARN on a NULL shadow page in TDP MMU check

Treat a NULL shadow page in the "is a TDP MMU" check as valid, non-TDP
root.  KVM uses a "direct" PAE paging MMU when TDP is disabled and the
guest is running with paging disabled.  In that case, root_hpa points at
the pae_root page (of which only 32 bytes are used), not a standard
shadow page, and the WARN fires (a lot).

Fixes: 0b873fd7fb53 ("KVM: x86/mmu: Remove redundant is_tdp_mmu_enabled check")
Cc: David Matlack <dmatlack@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622072454.3449146-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
index b981a04..1cae448 100644 (file)
--- a/arch/x86/kvm/mmu/tdp_mmu.h
+++ b/arch/x86/kvm/mmu/tdp_mmu.h
@@ -94,11 +94,13 @@ static inline bool is_tdp_mmu(struct kvm_mmu *mmu)
        if (WARN_ON(!VALID_PAGE(hpa)))
                return false;
 
+       /*
+        * A NULL shadow page is legal when shadowing a non-paging guest with
+        * PAE paging, as the MMU will be direct with root_hpa pointing at the
+        * pae_root page, not a shadow page.
+        */
        sp = to_shadow_page(hpa);
-       if (WARN_ON(!sp))
-               return false;
-
-       return is_tdp_mmu_page(sp) && sp->root_count;
+       return sp && is_tdp_mmu_page(sp) && sp->root_count;
 }
 #else
 static inline bool kvm_mmu_init_tdp_mmu(struct kvm *kvm) { return false; }