[RFC,23/24] KVM: nSVM: Allocate a new ASID for nested guests

Commit Message

Yosry Ahmed March 26, 2025, 7:44 p.m. UTC

Now that nested TLB flushes are properly tracked, start allocating a
separate ASID for nested guests. This allows dropping the unconditional
TLB flushes on nested transitions and doing finer grained TLB flushing
when necessary.

Signed-off-by: Yosry Ahmed <[e-mail]>
---
 arch/x86/kvm/svm/nested.c | 11 +++++++++--
 arch/x86/kvm/svm/svm.c    |  5 +++--
 arch/x86/kvm/svm/svm.h    |  3 +++
 3 files changed, 15 insertions(+), 4 deletions(-)

Patch

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 544913461693c..0c887c91bd50d 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -1204,6 +1204,7 @@  int svm_allocate_nested(struct vcpu_svm *svm)
 {
 	struct kvm_svm *kvm_svm = to_kvm_svm(svm->vcpu.kvm);
 	struct page *vmcb02_page;
+	unsigned int asid;
 
 	if (svm->nested.initialized)
 		return 0;
@@ -1221,8 +1222,14 @@  int svm_allocate_nested(struct vcpu_svm *svm)
 
 	svm->nested.initialized = true;
 
-	if (!kvm_svm->nested_asid)
-		kvm_svm->nested_asid = kvm_svm->asid;
+	if (!kvm_svm->nested_asid) {
+		asid = kvm_tlb_tags_alloc(&svm_asids);
+		if (asid && !svm_register_asid(asid)) {
+			kvm_tlb_tags_free(&svm_asids, asid);
+			asid = 0;
+		}
+		kvm_svm->nested_asid = asid ?: fallback_asid;
+	}
 
 	return 0;
 
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 4b95fd6b501e6..196f5bca57a0e 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -249,8 +249,8 @@  static unsigned long iopm_base;
 
 DEFINE_PER_CPU(struct svm_cpu_data, svm_data);
 
-static struct kvm_tlb_tags svm_asids;
-static unsigned int fallback_asid;
+struct kvm_tlb_tags svm_asids;
+unsigned int fallback_asid;
 
 /*
  * Only MSR_TSC_AUX is switched via the user return hook.  EFER is switched via
@@ -5127,6 +5127,7 @@  static void svm_vm_destroy(struct kvm *kvm)
 	avic_vm_destroy(kvm);
 	sev_vm_destroy(kvm);
 	kvm_tlb_tags_free(&svm_asids, kvm_svm->asid);
+	kvm_tlb_tags_free(&svm_asids, kvm_svm->nested_asid);
 }
 
 static int svm_vm_init(struct kvm *kvm)
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 0c44133bc05ca..220d10d2b1a5c 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -630,6 +630,9 @@  static inline void svm_vmgexit_no_action(struct vcpu_svm *svm, u64 data)
 
 extern bool dump_invalid_vmcb;
 
+extern struct kvm_tlb_tags svm_asids;
+extern unsigned int fallback_asid;
+
 u32 svm_msrpm_offset(u32 msr);
 u32 *svm_vcpu_alloc_msrpm(void);
 void svm_vcpu_init_msrpm(struct kvm_vcpu *vcpu, u32 *msrpm);