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<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.