linux-kernel - [PATCH v4 10/11] KVM: x86/mmu: Use static key/branches for checking if TDP MMU is enabled

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Date:   Wed, 12 Oct 2022 18:17:01 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        David Matlack <dmatlack@...gle.com>,
        Isaku Yamahata <isaku.yamahata@...el.com>
Subject: [PATCH v4 10/11] KVM: x86/mmu: Use static key/branches for checking
 if TDP MMU is enabled

Now that the TDP MMU being enabled is read-only after the vendor module
is loaded, use a static key to track whether or not the TDP MMU is
enabled to avoid conditional branches in hot paths, e.g. in
direct_page_fault() and fast_page_fault().

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/mmu.h     |  5 +++--
 arch/x86/kvm/mmu/mmu.c | 14 ++++++++++----
 2 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index 1ad6d02e103f..bc0d8a5c09f9 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -2,6 +2,7 @@
 #ifndef __KVM_X86_MMU_H
 #define __KVM_X86_MMU_H
 
+#include <linux/jump_label.h>
 #include <linux/kvm_host.h>
 #include "kvm_cache_regs.h"
 #include "cpuid.h"
@@ -230,13 +231,13 @@ static inline bool kvm_shadow_root_allocated(struct kvm *kvm)
 }
 
 #ifdef CONFIG_X86_64
-extern bool tdp_mmu_enabled;
+DECLARE_STATIC_KEY_TRUE(tdp_mmu_enabled);
 #endif
 
 static inline bool is_tdp_mmu_enabled(void)
 {
 #ifdef CONFIG_X86_64
-	return tdp_mmu_enabled;
+	return static_branch_likely(&tdp_mmu_enabled);
 #else
 	return false;
 #endif
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 4792d76edd6d..a5ba7b41263d 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -101,8 +101,10 @@ bool tdp_enabled = false;
 #ifdef CONFIG_X86_64
 static bool __ro_after_init tdp_mmu_allowed;
 
-bool __read_mostly tdp_mmu_enabled = true;
-module_param_named(tdp_mmu, tdp_mmu_enabled, bool, 0444);
+static bool __read_mostly __tdp_mmu_enabled = true;
+module_param_named(tdp_mmu, __tdp_mmu_enabled, bool, 0444);
+
+DEFINE_STATIC_KEY_TRUE(tdp_mmu_enabled);
 #endif
 
 static int max_huge_page_level __read_mostly;
@@ -5702,7 +5704,11 @@ void kvm_configure_mmu(bool enable_tdp, int tdp_forced_root_level,
 	max_tdp_level = tdp_max_root_level;
 
 #ifdef CONFIG_X86_64
-	tdp_mmu_enabled = tdp_mmu_allowed && tdp_enabled;
+	__tdp_mmu_enabled = tdp_mmu_allowed && tdp_enabled;
+	if (__tdp_mmu_enabled)
+		static_branch_enable(&tdp_mmu_enabled);
+	else
+		static_branch_disable(&tdp_mmu_enabled);
 #endif
 	/*
 	 * max_huge_page_level reflects KVM's MMU capabilities irrespective
@@ -6712,7 +6718,7 @@ void __init kvm_mmu_x86_module_init(void)
 	 * TDP MMU is actually enabled is determined in kvm_configure_mmu()
 	 * when the vendor module is loaded.
 	 */
-	tdp_mmu_allowed = tdp_mmu_enabled;
+	tdp_mmu_allowed = __tdp_mmu_enabled;
 #endif
 
 	kvm_mmu_spte_module_init();
-- 
2.38.0.rc1.362.ged0d419d3c-goog