On 5/1/2025 11:30 AM, Sean Christopherson wrote:
From c50fb5a8a46058bbcfdcac0a100c2aa0f7f68f1c Mon Sep 17 00:00:00 2001
From: Sean Christopherson<seanjc@xxxxxxxxxx>
Date: Thu, 1 May 2025 11:10:39 -0700
Subject: [PATCH 2/2] x86/fred: KVM: VMX: Always use FRED for IRQ+NMI when
CONFIG_X86_FRED=y
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
I LOVE this change, but not sure if everyone is happy with it.
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))
"if (IS_ENABLED(CONFIG_X86_64))"?
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))
Ditto.
