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

H. Peter Anvin
2025-05-02T19:44:28Z

On May 1, 2025 10:48:42 PM PDT, Xin Li <xin@zytor.com> wrote:
>On 5/1/2025 11:30 AM, Sean Christopherson wrote:
>> From c50fb5a8a46058bbcfdcac0a100c2aa0f7f68f1c Mon Sep 17 00:00:00 2001
>> From: Sean Christopherson<seanjc@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<peterz@infradead.org>
>> Signed-off-by: Sean Christopherson<seanjc@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.

I don't think anyone will have a problem with compiling it in... it is such a small amount of code.