[PATCH 1/7] KVM: x86: Make vmx_get_exit_qual() and vmx_get_intr_info() noinstr-friendly

Sean Christopherson
2022-12-13T06:09:31Z

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>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/kvm_cache_regs.h | 12 ++++++++++++
arch/x86/kvm/vmx/vmx.h | 14 ++++++--------
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
@@ -75,6 +75,18 @@ static inline void kvm_register_mark_dirty(struct kvm_vcpu *vcpu,
__set_bit(reg, (unsigned long *)&vcpu->arch.regs_dirty);
}

+/*
+ * 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,
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;
}

--
2.39.0.rc1.256.g54fd8350bd-goog