[PATCH v2 10/25] KVM: x86/mmu: remove ept_ad field

Paolo Bonzini
2022-02-21T16:23:34Z

The ept_ad field is used during page walk to determine if the guest PTEs
have accessed and dirty bits. In the MMU role, the ad_disabled
bit represents whether the *shadow* PTEs have the bits, so it
would be incorrect to replace PT_HAVE_ACCESSED_DIRTY with just
!mmu->mmu_role.base.ad_disabled.

However, the similar field in the CPU mode, ad_disabled, is initialized
correctly: to the opposite value of ept_ad for shadow EPT, and zero
for non-EPT guest paging modes (which always have A/D bits). It is
therefore possible to compute PT_HAVE_ACCESSED_DIRTY from the CPU mode,
like other page-format fields; it just has to be inverted to account
for the different polarity.

Having a CPU mode that is distinct from the MMU roles in fact would even
allow to remove PT_HAVE_ACCESSED_DIRTY macro altogether, and always use
!mmu->cpu_mode.base.ad_disabled. I am not doing this because the macro
has a small effect in terms of dead code elimination:

text data bss dec hex
103544 16665 112 120321 1d601 # as of this patch
103746 16665 112 120523 1d6cb # without PT_HAVE_ACCESSED_DIRTY

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
arch/x86/include/asm/kvm_host.h | 1 -
arch/x86/kvm/mmu/mmu.c | 1 -
arch/x86/kvm/mmu/paging_tmpl.h | 2 +-
3 files changed, 1 insertion(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index cc268116eb3f..996cf9b14f5e 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -437,7 +437,6 @@ struct kvm_mmu {
union kvm_mmu_role mmu_role;
u8 root_level;
u8 shadow_root_level;
- u8 ept_ad;
bool direct_map;
struct kvm_mmu_root_info prev_roots[KVM_MMU_NUM_PREV_ROOTS];

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 6e539fc2c9c7..3ffa6f2bf991 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4936,7 +4936,6 @@ void kvm_init_shadow_ept_mmu(struct kvm_vcpu *vcpu, bool execonly,

context->shadow_root_level = level;

- context->ept_ad = accessed_dirty;
context->page_fault = ept_page_fault;
context->gva_to_gpa = ept_gva_to_gpa;
context->sync_page = ept_sync_page;
diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index e1c2ecb4ddee..64b6f76641f0 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -64,7 +64,7 @@
#define PT_LEVEL_BITS PT64_LEVEL_BITS
#define PT_GUEST_DIRTY_SHIFT 9
#define PT_GUEST_ACCESSED_SHIFT 8
- #define PT_HAVE_ACCESSED_DIRTY(mmu) ((mmu)->ept_ad)
+ #define PT_HAVE_ACCESSED_DIRTY(mmu) (!(mmu)->cpu_mode.base.ad_disabled)
#define CMPXCHG cmpxchg64
#define PT_MAX_FULL_LEVELS PT64_ROOT_MAX_LEVEL
#else
--
2.31.1