KVM: x86/mmu: Use a dedicated bit to track shadow/MMU-present SPTEs

Introduce MMU_PRESENT to explicitly track which SPTEs are "present" from
the MMU's perspective.  Checking for shadow-present SPTEs is a very
common operation for the MMU, particularly in hot paths such as page
faults.  With the addition of "removed" SPTEs for the TDP MMU,
identifying shadow-present SPTEs is quite costly especially since it
requires checking multiple 64-bit values.

On 64-bit KVM, this reduces the footprint of kvm.ko's .text by ~2k bytes.
On 32-bit KVM, this increases the footprint by ~200 bytes, but only
because gcc now inlines several more MMU helpers, e.g. drop_parent_pte().

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210225204749.1512652-21-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index d12acf5eb87125cd1aff7b6d4dd386d79c32aa51..e07aabb23b8a6db50eceff3e02daad9e5f3c1b33 100644 (file)
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -94,7 +94,7 @@ int make_spte(struct kvm_vcpu *vcpu, unsigned int pte_access, int level,
                     bool can_unsync, bool host_writable, bool ad_disabled,
                     u64 *new_spte)
 {
-       u64 spte = 0;
+       u64 spte = SPTE_MMU_PRESENT_MASK;
        int ret = 0;
 
        if (ad_disabled)
@@ -183,10 +183,10 @@ out:
 
 u64 make_nonleaf_spte(u64 *child_pt, bool ad_disabled)
 {
-       u64 spte;
+       u64 spte = SPTE_MMU_PRESENT_MASK;
 
-       spte = __pa(child_pt) | shadow_present_mask | PT_WRITABLE_MASK |
-              shadow_user_mask | shadow_x_mask | shadow_me_mask;
+       spte |= __pa(child_pt) | shadow_present_mask | PT_WRITABLE_MASK |
+               shadow_user_mask | shadow_x_mask | shadow_me_mask;
 
        if (ad_disabled)
                spte |= SPTE_TDP_AD_DISABLED_MASK;

diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
index 8996baa8da1508cbdf41f7bee4ee993633449e5c..645e9bc2d4a295b0702d8eb2d62eed270d25851e 100644 (file)
--- a/arch/x86/kvm/mmu/spte.h
+++ b/arch/x86/kvm/mmu/spte.h
@@ -5,6 +5,15 @@
 
 #include "mmu_internal.h"
 
+/*
+ * A MMU present SPTE is backed by actual memory and may or may not be present
+ * in hardware.  E.g. MMIO SPTEs are not considered present.  Use bit 11, as it
+ * is ignored by all flavors of SPTEs and checking a low bit often generates
+ * better code than for a high bit, e.g. 56+.  MMU present checks are pervasive
+ * enough that the improved code generation is noticeable in KVM's footprint.
+ */
+#define SPTE_MMU_PRESENT_MASK          BIT_ULL(11)
+
 /*
  * TDP SPTES (more specifically, EPT SPTEs) may not have A/D bits, and may also
  * be restricted to using write-protection (for L2 when CPU dirty logging, i.e.
@@ -241,7 +250,7 @@ static inline bool is_access_track_spte(u64 spte)
 
 static inline bool is_shadow_present_pte(u64 pte)
 {
-       return (pte != 0) && !is_mmio_spte(pte) && !is_removed_spte(pte);
+       return !!(pte & SPTE_MMU_PRESENT_MASK);
 }
 
 static inline bool is_large_pte(u64 pte)