[PATCH v16 08/51] KVM: x86: Initialize kvm_caps.supported_xss

Sean Christopherson
2025-09-19T22:33:17Z

From: Yang Weijiang <weijiang.yang@intel.com>

Set original kvm_caps.supported_xss to (host_xss & KVM_SUPPORTED_XSS) if
XSAVES is supported. host_xss contains the host supported xstate feature
bits for thread FPU context switch, KVM_SUPPORTED_XSS includes all KVM
enabled XSS feature bits, the resulting value represents the supervisor
xstates that are available to guest and are backed by host FPU framework
for swapping {guest,host} XSAVE-managed registers/MSRs.

[sean: relocate and enhance comment about PT / XSS[8] ]

Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/x86.c | 23 +++++++++++++++--------
1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c87ed216f72a..3e66d8c5000a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -217,6 +217,14 @@ static struct kvm_user_return_msrs __percpu *user_return_msrs;
| XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
| XFEATURE_MASK_PKRU | XFEATURE_MASK_XTILE)

+/*
+ * Note, KVM supports exposing PT to the guest, but does not support context
+ * switching PT via XSTATE (KVM's PT virtualization relies on perf; swapping
+ * PT via guest XSTATE would clobber perf state), i.e. KVM doesn't support
+ * IA32_XSS[bit 8] (guests can/must use RDMSR/WRMSR to save/restore PT MSRs).
+ */
+#define KVM_SUPPORTED_XSS 0
+
bool __read_mostly allow_smaller_maxphyaddr = 0;
EXPORT_SYMBOL_GPL(allow_smaller_maxphyaddr);

@@ -3986,11 +3994,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
case MSR_IA32_XSS:
if (!guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
return KVM_MSR_RET_UNSUPPORTED;
- /*
- * KVM supports exposing PT to the guest, but does not support
- * IA32_XSS[bit 8]. Guests have to use RDMSR/WRMSR rather than
- * XSAVES/XRSTORS to save/restore PT MSRs.
- */
+
if (data & ~vcpu->arch.guest_supported_xss)
return 1;
if (vcpu->arch.ia32_xss == data)
@@ -9822,14 +9826,17 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
kvm_host.xcr0 = xgetbv(XCR_XFEATURE_ENABLED_MASK);
kvm_caps.supported_xcr0 = kvm_host.xcr0 & KVM_SUPPORTED_XCR0;
}
+
+ if (boot_cpu_has(X86_FEATURE_XSAVES)) {
+ rdmsrq(MSR_IA32_XSS, kvm_host.xss);
+ kvm_caps.supported_xss = kvm_host.xss & KVM_SUPPORTED_XSS;
+ }
+
kvm_caps.supported_quirks = KVM_X86_VALID_QUIRKS;
kvm_caps.inapplicable_quirks = KVM_X86_CONDITIONAL_QUIRKS;

rdmsrq_safe(MSR_EFER, &kvm_host.efer);

- if (boot_cpu_has(X86_FEATURE_XSAVES))
- rdmsrq(MSR_IA32_XSS, kvm_host.xss);
-
kvm_init_pmu_capability(ops->pmu_ops);

if (boot_cpu_has(X86_FEATURE_ARCH_CAPABILITIES))
--
2.51.0.470.ga7dc726c21-goog