linux-kernel - [PATCH 20/24] KVM: x86/mmu: Use a dedicated bit to track shadow/MMU-present SPTEs

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Message-Id: <20210225204749.1512652-21-seanjc@google.com>
Date:   Thu, 25 Feb 2021 12:47:45 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>
Subject: [PATCH 20/24] 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@...gle.com>
---
 arch/x86/kvm/mmu/spte.c |  8 ++++----
 arch/x86/kvm/mmu/spte.h | 11 ++++++++++-
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index d12acf5eb871..e07aabb23b8a 100644
--- 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 @@ 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;
diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
index 8996baa8da15..645e9bc2d4a2 100644
--- 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)
-- 
2.30.1.766.gb4fecdf3b7-goog