KVM: x86: Make vmx_get_exit_qual() and vmx_get_intr_info() noinstr-friendly

Add an extra special noinstr-friendly helper to test+mark a "register"
available and use it when caching vmcs.EXIT_QUALIFICATION and
vmcs.VM_EXIT_INTR_INFO.  Make the caching helpers __always_inline too so
that they can be used in noinstr functions.

A future fix will move VMX's handling of NMI exits into the noinstr
vmx_vcpu_enter_exit() so that the NMI is processed before any kind of
instrumentation can trigger a fault and thus IRET, i.e. so that KVM
doesn't invoke the NMI handler with NMIs enabled.

Cc: Peter Zijlstra <peterz@infradead.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20221213060912.654668-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

2 files changed, 18 insertions, 8 deletions

diff --git a/arch/x86/kvm/kvm_cache_regs.h b/arch/x86/kvm/kvm_cache_regs.h
index c09174f73a34..4c91f626c058 100644
--- a/arch/x86/kvm/kvm_cache_regs.h
+++ b/arch/x86/kvm/kvm_cache_regs.h
@@ -76,6 +76,18 @@ static inline void kvm_register_mark_dirty(struct kvm_vcpu *vcpu,
 }
 
 /*
+ * kvm_register_test_and_mark_available() is a special snowflake that uses an
+ * arch bitop directly to avoid the explicit instrumentation that comes with
+ * the generic bitops.  This allows code that cannot be instrumented (noinstr
+ * functions), e.g. the low level VM-Enter/VM-Exit paths, to cache registers.
+ */
+static __always_inline bool kvm_register_test_and_mark_available(struct kvm_vcpu *vcpu,
+								 enum kvm_reg reg)
+{
+	return arch___test_and_set_bit(reg, (unsigned long *)&vcpu->arch.regs_avail);
+}
+
+/*
  * The "raw" register helpers are only for cases where the full 64 bits of a
  * register are read/written irrespective of current vCPU mode.  In other words,
  * odds are good you shouldn't be using the raw variants.
diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index a3da84f4ea45..bb720a2f11ab 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -669,25 +669,23 @@ void intel_pmu_cross_mapped_check(struct kvm_pmu *pmu);
 int intel_pmu_create_guest_lbr_event(struct kvm_vcpu *vcpu);
 void vmx_passthrough_lbr_msrs(struct kvm_vcpu *vcpu);
 
-static inline unsigned long vmx_get_exit_qual(struct kvm_vcpu *vcpu)
+static __always_inline unsigned long vmx_get_exit_qual(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 
-	if (!kvm_register_is_available(vcpu, VCPU_EXREG_EXIT_INFO_1)) {
-		kvm_register_mark_available(vcpu, VCPU_EXREG_EXIT_INFO_1);
+	if (!kvm_register_test_and_mark_available(vcpu, VCPU_EXREG_EXIT_INFO_1))
 		vmx->exit_qualification = vmcs_readl(EXIT_QUALIFICATION);
-	}
+
 	return vmx->exit_qualification;
 }
 
-static inline u32 vmx_get_intr_info(struct kvm_vcpu *vcpu)
+static __always_inline u32 vmx_get_intr_info(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 
-	if (!kvm_register_is_available(vcpu, VCPU_EXREG_EXIT_INFO_2)) {
-		kvm_register_mark_available(vcpu, VCPU_EXREG_EXIT_INFO_2);
+	if (!kvm_register_test_and_mark_available(vcpu, VCPU_EXREG_EXIT_INFO_2))
 		vmx->exit_intr_info = vmcs_read32(VM_EXIT_INTR_INFO);
-	}
+
 	return vmx->exit_intr_info;
 }