[PATCH v2 09/11] KVM: x86/MMU: Allow NX huge pages to be disabled on a per-vm basis

[Ben Gardon <bgardon@google.com>]

In some cases, the NX hugepage mitigation for iTLB multihit is not
needed for all guests on a host. Allow disabling the mitigation on a
per-VM basis to avoid the performance hit of NX hugepages on trusted
workloads.

Signed-off-by: Ben Gardon <bgardon@google.com>
---
 arch/x86/include/asm/kvm_host.h | 1 +
 arch/x86/kvm/mmu.h              | 1 +
 arch/x86/kvm/mmu/mmu.c          | 6 ++++--
 arch/x86/kvm/x86.c              | 6 ++++++
 include/uapi/linux/kvm.h        | 1 +
 5 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 0a0c54639dd8..04ddfc475ce0 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1242,6 +1242,7 @@ struct kvm_arch {
 #endif
 
 	bool nx_huge_pages;
+	bool disable_nx_huge_pages;
 };
 
 struct kvm_vm_stat {
diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index dd28fe8d13ae..36d8d84ca6c6 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -177,6 +177,7 @@ static inline bool is_nx_huge_page_enabled(struct kvm *kvm)
 {
 	return READ_ONCE(kvm->arch.nx_huge_pages);
 }
+void kvm_update_nx_huge_pages(struct kvm *kvm);
 
 static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 					u32 err, bool prefetch)
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index dc9672f70468..a7d387ccfd74 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -6195,9 +6195,10 @@ static void __set_nx_huge_pages(bool val)
 	nx_huge_pages = itlb_multihit_kvm_mitigation = val;
 }
 
-static void kvm_update_nx_huge_pages(struct kvm *kvm)
+void kvm_update_nx_huge_pages(struct kvm *kvm)
 {
-	kvm->arch.nx_huge_pages = nx_huge_pages;
+	kvm->arch.nx_huge_pages = nx_huge_pages &&
+				  !kvm->arch.disable_nx_huge_pages;
 
 	mutex_lock(&kvm->slots_lock);
 	kvm_mmu_zap_all_fast(kvm);
@@ -6451,6 +6452,7 @@ int kvm_mmu_post_init_vm(struct kvm *kvm)
 	int err;
 
 	kvm->arch.nx_huge_pages = READ_ONCE(nx_huge_pages);
+	kvm->arch.disable_nx_huge_pages = false;
 	err = kvm_vm_create_worker_thread(kvm, kvm_nx_lpage_recovery_worker, 0,
 					  "kvm-nx-lpage-recovery",
 					  &kvm->arch.nx_lpage_recovery_thread);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 51106d32f04e..73df90a6932b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4256,6 +4256,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_SYS_ATTRIBUTES:
 	case KVM_CAP_VAPIC:
 	case KVM_CAP_ENABLE_CAP:
+	case KVM_CAP_VM_DISABLE_NX_HUGE_PAGES:
 		r = 1;
 		break;
 	case KVM_CAP_EXIT_HYPERCALL:
@@ -6048,6 +6049,11 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 		}
 		mutex_unlock(&kvm->lock);
 		break;
+	case KVM_CAP_VM_DISABLE_NX_HUGE_PAGES:
+		kvm->arch.disable_nx_huge_pages = true;
+		kvm_update_nx_huge_pages(kvm);
+		r = 0;
+		break;
 	default:
 		r = -EINVAL;
 		break;
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index ee5cc9e2a837..6f9fa7ecfd1e 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1144,6 +1144,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_S390_MEM_OP_EXTENSION 211
 #define KVM_CAP_PMU_CAPABILITY 212
 #define KVM_CAP_DISABLE_QUIRKS2 213
+#define KVM_CAP_VM_DISABLE_NX_HUGE_PAGES 214
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
-- 
2.35.1.894.gb6a874cedc-goog