linux-kernel - [PATCH v2 16/18] KVM: x86: introduce KVM_REQ_MMU_UPDATE

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Date:   Thu, 17 Feb 2022 16:03:38 -0500
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     seanjc@...gle.com
Subject: [PATCH v2 16/18] KVM: x86: introduce KVM_REQ_MMU_UPDATE_ROOT

Whenever KVM knows the page role flags have changed, it needs to drop
the current MMU root and possibly load one from the prev_roots cache.
Currently it is papering over some overly simplistic code by just
dropping _all_ roots, so that the root will be reloaded by
kvm_mmu_reload, but this has bad performance for the TDP MMU
(which drops the whole of the page tables when freeing a root,
without the performance safety net of a hash table).

To do this, KVM needs to do a more kvm_mmu_update_root call from
kvm_mmu_reset_context.  Introduce a new request bit so that the call
can be delayed until after a possible KVM_REQ_MMU_RELOAD, which would
kill all hopes of finding a cached PGD.

Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/include/asm/kvm_host.h |  1 +
 arch/x86/kvm/svm/nested.c       |  2 +-
 arch/x86/kvm/vmx/nested.c       |  2 +-
 arch/x86/kvm/x86.c              | 13 +++++++++++--
 4 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 319ac0918aa2..532cda546eb9 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -102,6 +102,7 @@
 #define KVM_REQ_MSR_FILTER_CHANGED	KVM_ARCH_REQ(29)
 #define KVM_REQ_UPDATE_CPU_DIRTY_LOGGING \
 	KVM_ARCH_REQ_FLAGS(30, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
+#define KVM_REQ_MMU_UPDATE_ROOT		KVM_ARCH_REQ(31)
 
 #define CR0_RESERVED_BITS                                               \
 	(~(unsigned long)(X86_CR0_PE | X86_CR0_MP | X86_CR0_EM | X86_CR0_TS \
diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 2386fadae9ed..8e6e62d8df36 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -498,7 +498,7 @@ static int nested_svm_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3,
 	kvm_init_mmu(vcpu);
 
 	if (!nested_npt)
-		kvm_mmu_update_root(vcpu);
+		kvm_make_request(KVM_REQ_MMU_UPDATE_ROOT, vcpu);
 
 	return 0;
 }
diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 2dbd7a9ada84..c3595bc0a02d 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -1133,7 +1133,7 @@ static int nested_vmx_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3,
 	kvm_init_mmu(vcpu);
 
 	if (!nested_ept)
-		kvm_mmu_update_root(vcpu);
+		kvm_make_request(KVM_REQ_MMU_UPDATE_ROOT, vcpu);
 
 	return 0;
 }
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 9800c8883a48..9043548e6baf 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1189,7 +1189,7 @@ int kvm_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
 		return 1;
 
 	if (cr3 != kvm_read_cr3(vcpu))
-		kvm_mmu_update_root(vcpu);
+		kvm_make_request(KVM_REQ_MMU_UPDATE_ROOT, vcpu);
 
 	vcpu->arch.cr3 = cr3;
 	kvm_register_mark_dirty(vcpu, VCPU_EXREG_CR3);
@@ -9835,8 +9835,15 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 				goto out;
 			}
 		}
-		if (kvm_check_request(KVM_REQ_MMU_RELOAD, vcpu))
+		if (kvm_check_request(KVM_REQ_MMU_RELOAD, vcpu)) {
 			kvm_mmu_unload(vcpu);
+
+			/*
+			 * Dropping all roots leaves no hope for loading a cached
+			 * one.  Let kvm_mmu_reload build a new one.
+			 */
+			kvm_clear_request(KVM_REQ_MMU_UPDATE_ROOT, vcpu);
+		}
 		if (kvm_check_request(KVM_REQ_MIGRATE_TIMER, vcpu))
 			__kvm_migrate_timers(vcpu);
 		if (kvm_check_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu))
@@ -9848,6 +9855,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 			if (unlikely(r))
 				goto out;
 		}
+		if (kvm_check_request(KVM_REQ_MMU_UPDATE_ROOT, vcpu))
+			kvm_mmu_update_root(vcpu);
 		if (kvm_check_request(KVM_REQ_MMU_SYNC, vcpu))
 			kvm_mmu_sync_roots(vcpu);
 		if (kvm_check_request(KVM_REQ_LOAD_MMU_PGD, vcpu))
-- 
2.31.1