[PATCH 2/2] x86/fred: KVM: VMX: Always use FRED for IRQ+NMI when CONFIG_X86_FRED=y

[Sean Christopherson]

Now that FRED provides C-code entry points for handling IRQ and NMI exits,
use the FRED infrastructure for forwarding all such events even if FRED
isn't supported in hardware.  Avoiding the non-FRED assembly trampolines
into the IDT handlers for IRQs eliminates the associated non-CFI indirect
call (KVM performs a CALL by doing a lookup on the IDT using the IRQ
vector).

Force FRED for 64-bit kernels if KVM_INTEL is enabled, as the benefits of
eliminating the IRQ trampoline usage far outwieghts the code overhead for
FRED.

Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kvm/Kconfig   | 1 +
 arch/x86/kvm/vmx/vmx.c | 4 ++--
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index 2eeffcec5382..712a2ff28ce4 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -95,6 +95,7 @@ config KVM_SW_PROTECTED_VM
 config KVM_INTEL
 	tristate "KVM for Intel (and compatible) processors support"
 	depends on KVM && IA32_FEAT_CTL
+	select X86_FRED if X86_64
 	select KVM_GENERIC_PRIVATE_MEM if INTEL_TDX_HOST
 	select KVM_GENERIC_MEMORY_ATTRIBUTES if INTEL_TDX_HOST
 	help
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index ef2d7208dd20..2ea89985107d 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6995,7 +6995,7 @@ static void handle_external_interrupt_irqoff(struct kvm_vcpu *vcpu,
 		return;
 
 	kvm_before_interrupt(vcpu, KVM_HANDLING_IRQ);
-	if (cpu_feature_enabled(X86_FEATURE_FRED))
+	if (IS_ENABLED(CONFIG_X86_FRED))
 		fred_entry_from_kvm(EVENT_TYPE_EXTINT, vector);
 	else
 		vmx_do_interrupt_irqoff(gate_offset((gate_desc *)host_idt_base + vector));
@@ -7268,7 +7268,7 @@ noinstr void vmx_handle_nmi(struct kvm_vcpu *vcpu)
 		return;
 
 	kvm_before_interrupt(vcpu, KVM_HANDLING_NMI);
-	if (cpu_feature_enabled(X86_FEATURE_FRED))
+	if (IS_ENABLED(CONFIG_X86_FRED))
 		fred_entry_from_kvm(EVENT_TYPE_NMI, NMI_VECTOR);
 	else
 		vmx_do_nmi_irqoff();
-- 
2.49.0.906.g1f30a19c02-goog