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

From: Yosry Ahmed
Date: Wed Mar 26 2025 - 15:49:50 EST


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 <yosry.ahmed@xxxxxxxxx>
---
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(-)

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);
--
2.49.0.395.g12beb8f557-goog