KVM: x86: Snapshot host's MSR_IA32_ARCH_CAPABILITIES

Snapshot the host's MSR_IA32_ARCH_CAPABILITIES, if it's supported, instead
of reading the MSR every time KVM wants to query the host state, e.g. when
initializing the default value during vCPU creation.  The paths that query
ARCH_CAPABILITIES aren't particularly performance sensitive, but creating
vCPUs is a frequent enough operation that burning 8 bytes is a good
trade-off.

Alternatively, KVM could add a field in kvm_caps and thus skip the
on-demand calculations entirely, but a pure snapshot isn't possible due to
the way KVM handles the l1tf_vmx_mitigation module param.  And unlike the
other "supported" fields in kvm_caps, KVM doesn't enforce the "supported"
value, i.e. KVM treats ARCH_CAPABILITIES like a CPUID leaf and lets
userspace advertise whatever it wants.  Those problems are solvable, but
it's not clear there is real benefit versus snapshotting the host value,
and grabbing the host value will allow additional cleanup of KVM's
FB_CLEAR_CTRL code.

Link: https://lore.kernel.org/all/20230524061634.54141-2-chao.gao@intel.com
Cc: Chao Gao <chao.gao@intel.com>
Cc: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20230607004311.1420507-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

3 files changed, 14 insertions, 22 deletions

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 254f2296e549..f0ec9acae86c 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -255,14 +255,9 @@ static int vmx_setup_l1d_flush(enum vmx_l1d_flush_state l1tf)
 		return 0;
 	}
 
-	if (boot_cpu_has(X86_FEATURE_ARCH_CAPABILITIES)) {
-		u64 msr;
-
-		rdmsrl(MSR_IA32_ARCH_CAPABILITIES, msr);
-		if (msr & ARCH_CAP_SKIP_VMENTRY_L1DFLUSH) {
-			l1tf_vmx_mitigation = VMENTER_L1D_FLUSH_NOT_REQUIRED;
-			return 0;
-		}
+	if (host_arch_capabilities & ARCH_CAP_SKIP_VMENTRY_L1DFLUSH) {
+		l1tf_vmx_mitigation = VMENTER_L1D_FLUSH_NOT_REQUIRED;
+		return 0;
 	}
 
 	/* If set to auto use the default l1tf mitigation method */
@@ -373,15 +368,10 @@ static int vmentry_l1d_flush_get(char *s, const struct kernel_param *kp)
 
 static void vmx_setup_fb_clear_ctrl(void)
 {
-	u64 msr;
-
-	if (boot_cpu_has(X86_FEATURE_ARCH_CAPABILITIES) &&
+	if ((host_arch_capabilities & ARCH_CAP_FB_CLEAR_CTRL) &&
 	    !boot_cpu_has_bug(X86_BUG_MDS) &&
-	    !boot_cpu_has_bug(X86_BUG_TAA)) {
-		rdmsrl(MSR_IA32_ARCH_CAPABILITIES, msr);
-		if (msr & ARCH_CAP_FB_CLEAR_CTRL)
-			vmx_fb_clear_ctrl_available = true;
-	}
+	    !boot_cpu_has_bug(X86_BUG_TAA))
+		vmx_fb_clear_ctrl_available = true;
 }
 
 static __always_inline void vmx_disable_fb_clear(struct vcpu_vmx *vmx)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0fca1546e029..a1b13d2d1d71 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -237,6 +237,9 @@ EXPORT_SYMBOL_GPL(enable_apicv);
 u64 __read_mostly host_xss;
 EXPORT_SYMBOL_GPL(host_xss);
 
+u64 __read_mostly host_arch_capabilities;
+EXPORT_SYMBOL_GPL(host_arch_capabilities);
+
 const struct _kvm_stats_desc kvm_vm_stats_desc[] = {
 	KVM_GENERIC_VM_STATS(),
 	STATS_DESC_COUNTER(VM, mmu_shadow_zapped),
@@ -1611,12 +1614,7 @@ static bool kvm_is_immutable_feature_msr(u32 msr)
 
 static u64 kvm_get_arch_capabilities(void)
 {
-	u64 data = 0;
-
-	if (boot_cpu_has(X86_FEATURE_ARCH_CAPABILITIES)) {
-		rdmsrl(MSR_IA32_ARCH_CAPABILITIES, data);
-		data &= KVM_SUPPORTED_ARCH_CAP;
-	}
+	u64 data = host_arch_capabilities & KVM_SUPPORTED_ARCH_CAP;
 
 	/*
 	 * If nx_huge_pages is enabled, KVM's shadow paging will ensure that
@@ -9490,6 +9488,9 @@ static int __kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
 
 	kvm_init_pmu_capability(ops->pmu_ops);
 
+	if (boot_cpu_has(X86_FEATURE_ARCH_CAPABILITIES))
+		rdmsrl(MSR_IA32_ARCH_CAPABILITIES, host_arch_capabilities);
+
 	r = ops->hardware_setup();
 	if (r != 0)
 		goto out_mmu_exit;
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index 82e3dafc5453..1e7be1f6ab29 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -323,6 +323,7 @@ fastpath_t handle_fastpath_set_msr_irqoff(struct kvm_vcpu *vcpu);
 
 extern u64 host_xcr0;
 extern u64 host_xss;
+extern u64 host_arch_capabilities;
 
 extern struct kvm_caps kvm_caps;