[PATCH v5 17/19] KVM:x86: Enable guest CET supervisor xstate bit support

Yang Weijiang
2023-08-03T07:38:14Z

Add S_CET bit in kvm_caps.supported_xss so that guest can enumerate
the feature in CPUID(0xd,1).ECX.

Guest S_CET xstate bit is specially handled, i.e., it can be exposed
without related enabling on host side, because KVM manually saves/reloads
guest supervisor SHSTK SSPs and current XSS swap logic for host/guest aslo
supports doing so, thus it's safe to enable the bit without host support.

Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
---
arch/x86/kvm/x86.c | 8 +++++++-
arch/x86/kvm/x86.h | 2 +-
2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index aa92dec66f1e..2e200a5d00e9 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -230,7 +230,8 @@ static struct kvm_user_return_msrs __percpu *user_return_msrs;
| XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
| XFEATURE_MASK_PKRU | XFEATURE_MASK_XTILE)

-#define KVM_SUPPORTED_XSS (XFEATURE_MASK_CET_USER)
+#define KVM_SUPPORTED_XSS (XFEATURE_MASK_CET_USER | \
+ XFEATURE_MASK_CET_KERNEL)

u64 __read_mostly host_efer;
EXPORT_SYMBOL_GPL(host_efer);
@@ -9657,8 +9658,13 @@ static int __kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
rdmsrl_safe(MSR_EFER, &host_efer);

if (boot_cpu_has(X86_FEATURE_XSAVES)) {
+ u32 eax, ebx, ecx, edx;
+
+ cpuid_count(0xd, 1, &eax, &ebx, &ecx, &edx);
rdmsrl(MSR_IA32_XSS, host_xss);
kvm_caps.supported_xss = host_xss & KVM_SUPPORTED_XSS;
+ if (ecx & XFEATURE_MASK_CET_KERNEL)
+ kvm_caps.supported_xss |= XFEATURE_MASK_CET_KERNEL;
}

kvm_init_pmu_capability(ops->pmu_ops);
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index 373386fb9ed2..ea0ecb8f0df6 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -363,7 +363,7 @@ static inline bool kvm_mpx_supported(void)
== (XFEATURE_MASK_BNDREGS | XFEATURE_MASK_BNDCSR);
}

-#define CET_XSTATE_MASK (XFEATURE_MASK_CET_USER)
+#define CET_XSTATE_MASK (XFEATURE_MASK_CET_USER | XFEATURE_MASK_CET_KERNEL)
/*
* Shadow Stack and Indirect Branch Tracking feature enabling depends on
* whether host side CET user xstate bit is supported or not.
--
2.27.0