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().

We now need to drop bit 11, used for the MMU_PRESENT flag, from
the set of bits used to store the generation number in MMIO SPTEs.
Otherwise MMIO SPTEs with bit 11 set would get false positives for
is_shadow_present_spte() and lead to a variety of fireworks, from oopses
to likely hangs of the host kernel.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210225204749.1512652-21-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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 @@ int make_spte(struct kvm_vcpu *vcpu, unsigned int pte_access, int level,
 
 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;

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.
@@ -92,11 +101,11 @@ static_assert(!(EPT_SPTE_MMU_WRITABLE & SHADOW_ACC_TRACK_SAVED_MASK));
 #undef SHADOW_ACC_TRACK_SAVED_MASK
 
 /*
- * Due to limited space in PTEs, the MMIO generation is a 20 bit subset of
+ * Due to limited space in PTEs, the MMIO generation is a 19 bit subset of
  * the memslots generation and is derived as follows:
  *
- * Bits 0-8 of the MMIO generation are propagated to spte bits 3-11
- * Bits 9-19 of the MMIO generation are propagated to spte bits 52-62
+ * Bits 0-7 of the MMIO generation are propagated to spte bits 3-10
+ * Bits 8-18 of the MMIO generation are propagated to spte bits 52-62
  *
  * The KVM_MEMSLOT_GEN_UPDATE_IN_PROGRESS flag is intentionally not included in
  * the MMIO generation number, as doing so would require stealing a bit from
@@ -107,7 +116,7 @@ static_assert(!(EPT_SPTE_MMU_WRITABLE & SHADOW_ACC_TRACK_SAVED_MASK));
  */
 
 #define MMIO_SPTE_GEN_LOW_START		3
-#define MMIO_SPTE_GEN_LOW_END		11
+#define MMIO_SPTE_GEN_LOW_END		10
 
 #define MMIO_SPTE_GEN_HIGH_START	52
 #define MMIO_SPTE_GEN_HIGH_END		62
@@ -116,12 +125,14 @@ static_assert(!(EPT_SPTE_MMU_WRITABLE & SHADOW_ACC_TRACK_SAVED_MASK));
 						    MMIO_SPTE_GEN_LOW_START)
 #define MMIO_SPTE_GEN_HIGH_MASK		GENMASK_ULL(MMIO_SPTE_GEN_HIGH_END, \
 						    MMIO_SPTE_GEN_HIGH_START)
+static_assert(!(SPTE_MMU_PRESENT_MASK &
+		(MMIO_SPTE_GEN_LOW_MASK | MMIO_SPTE_GEN_HIGH_MASK)));
 
 #define MMIO_SPTE_GEN_LOW_BITS		(MMIO_SPTE_GEN_LOW_END - MMIO_SPTE_GEN_LOW_START + 1)
 #define MMIO_SPTE_GEN_HIGH_BITS		(MMIO_SPTE_GEN_HIGH_END - MMIO_SPTE_GEN_HIGH_START + 1)
 
 /* remember to adjust the comment above as well if you change these */
-static_assert(MMIO_SPTE_GEN_LOW_BITS == 9 && MMIO_SPTE_GEN_HIGH_BITS == 11);
+static_assert(MMIO_SPTE_GEN_LOW_BITS == 8 && MMIO_SPTE_GEN_HIGH_BITS == 11);
 
 #define MMIO_SPTE_GEN_LOW_SHIFT		(MMIO_SPTE_GEN_LOW_START - 0)
 #define MMIO_SPTE_GEN_HIGH_SHIFT	(MMIO_SPTE_GEN_HIGH_START - MMIO_SPTE_GEN_LOW_BITS)
@@ -241,7 +252,7 @@ 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)