KVM: x86/mmu: Use MMU role to check for matching guest page sizes

Originally, __kvm_sync_page used to check the cr4_pae bit in the role
to avoid zapping 4-byte kvm_mmu_pages when guest page size are 8-byte
or the other way round.  However, in commit 47c42e6b4192 ("KVM: x86: fix
handling of role.cr4_pae and rename it to 'gpte_size'", 2019-03-28) it
was observed that this did not work for nested EPT, where the page table
size would be 8 bytes even if CR4.PAE=0.  (Note that the check still
has to be done for nested *NPT*, so it is not possible to use tdp_enabled
or similar).

Therefore, a hack was introduced to identify nested EPT shadow pages
and unconditionally call __kvm_sync_page() on them.  However, it is
possible to do without the hack to identify nested EPT shadow pages:
if EPT is active, there will be no shadow pages in non-EPT format,
and all of them will have gpte_is_8_bytes set to true; we can just
check the MMU role directly, and the test will always be true.

Even for non-EPT shadow MMUs, this test should really always be true
now that __kvm_sync_page() is called if and only if the role is an
exact match (kvm_mmu_get_page()) or is part of the current MMU context
(kvm_mmu_sync_roots()).  A future commit will convert the likely-pointless
check into a meaningful WARN to enforce that the mmu_roles of the current
context and the shadow page are compatible.

Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622175739.3610207-11-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

2 files changed, 3 insertions, 16 deletions

diff --git a/Documentation/virt/kvm/mmu.rst b/Documentation/virt/kvm/mmu.rst
index 20d85daed395..ddbb23998742 100644
--- a/Documentation/virt/kvm/mmu.rst
+++ b/Documentation/virt/kvm/mmu.rst
@@ -192,9 +192,6 @@ Shadow pages contain the following information:
     Contains the value of cr4.smap && !cr0.wp for which the page is valid
     (pages for which this is true are different from other pages; see the
     treatment of cr0.wp=0 below).
-  role.ept_sp:
-    This is a virtual flag to denote a shadowed nested EPT page.  ept_sp
-    is true if "cr0_wp && smap_andnot_wp", an otherwise invalid combination.
   role.smm:
     Is 1 if the page is valid in system management mode.  This field
     determines which of the kvm_memslots array was used to build this
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 4af466f0ec6d..71a2ee755224 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -1780,16 +1780,13 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm,
 	  &(_kvm)->arch.mmu_page_hash[kvm_page_table_hashfn(_gfn)])	\
 		if ((_sp)->gfn != (_gfn) || (_sp)->role.direct) {} else
 
-static inline bool is_ept_sp(struct kvm_mmu_page *sp)
-{
-	return sp->role.cr0_wp && sp->role.smap_andnot_wp;
-}
-
 /* @sp->gfn should be write-protected at the call site */
 static bool __kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 			    struct list_head *invalid_list)
 {
-	if ((!is_ept_sp(sp) && sp->role.gpte_is_8_bytes != !!is_pae(vcpu)) ||
+	union kvm_mmu_page_role mmu_role = vcpu->arch.mmu->mmu_role.base;
+
+	if (sp->role.gpte_is_8_bytes != mmu_role.gpte_is_8_bytes ||
 	    vcpu->arch.mmu->sync_page(vcpu, sp) == 0) {
 		kvm_mmu_prepare_zap_page(vcpu->kvm, sp, invalid_list);
 		return false;
@@ -4721,13 +4718,6 @@ kvm_calc_shadow_ept_root_page_role(struct kvm_vcpu *vcpu, bool accessed_dirty,
 	role.base.guest_mode = true;
 	role.base.access = ACC_ALL;
 
-	/*
-	 * WP=1 and NOT_WP=1 is an impossible combination, use WP and the
-	 * SMAP variation to denote shadow EPT entries.
-	 */
-	role.base.cr0_wp = true;
-	role.base.smap_andnot_wp = true;
-
 	role.ext = kvm_calc_mmu_role_ext(vcpu);
 	role.ext.execonly = execonly;