Re: [PATCH v2 00/13] objtool: Detect and warn about indirect calls in __nocfi functions

[Xin Li]

On 5/1/2025 11:30 AM, Sean Christopherson wrote:

 From c50fb5a8a46058bbcfdcac0a100c2aa0f7f68f1c Mon Sep 17 00:00:00 2001
From: Sean Christopherson<sea...@google.com>
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<pet...@infradead.org>
Signed-off-by: Sean Christopherson<sea...@google.com>
---
  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.