linux-kernel - Re: [PATCH v4 07/11] KVM: VMX: Handle NMI Source report in VM exit

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Message-ID: <Zr9cwLUP-l2zgXtV@google.com>
Date: Fri, 16 Aug 2024 07:05:52 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc: X86 Kernel <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Dave Hansen <dave.hansen@...el.com>, 
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Xin Li <xin3.li@...el.com>, linux-perf-users@...r.kernel.org, 
	Peter Zijlstra <peterz@...radead.org>, Paolo Bonzini <pbonzini@...hat.com>, 
	Tony Luck <tony.luck@...el.com>, Andy Lutomirski <luto@...nel.org>, acme@...nel.org, 
	kan.liang@...ux.intel.com, Andi Kleen <andi.kleen@...el.com>, 
	Nikolay Borisov <nik.borisov@...e.com>, Sohil Mehta <sohil.mehta@...el.com>, 
	Zeng Guang <guang.zeng@...el.com>
Subject: Re: [PATCH v4 07/11] KVM: VMX: Handle NMI Source report in VM exit

On Tue, Jul 09, 2024, Jacob Pan wrote:
> From: Zeng Guang <guang.zeng@...el.com>
> 
> If the "NMI exiting" VM-execution control is 1, the value of the 16-bit NMI
> source vector is saved in the exit-qualification field in the VMCS when VM
> exits occur on CPUs that support NMI source.
> 
> KVM that is aware of NMI-source reporting will push the bitmask of NMI source
> vectors as the exceptoin event data field on the stack for then entry of FRED
> exception. Subsequently, the host NMI exception handler is invoked which
> will process NMI source information in the event data. This operation is
> independent of vCPU FRED enabling status.
> 
> Signed-off-by: Zeng Guang <guang.zeng@...el.com>
> Signed-off-by: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 4e7b36081b76..6719c598fa5f 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7331,10 +7331,15 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
>  	if ((u16)vmx->exit_reason.basic == EXIT_REASON_EXCEPTION_NMI &&
>  	    is_nmi(vmx_get_intr_info(vcpu))) {
>  		kvm_before_interrupt(vcpu, KVM_HANDLING_NMI);
> -		if (cpu_feature_enabled(X86_FEATURE_FRED))
> -			fred_entry_from_kvm(EVENT_TYPE_NMI, NMI_VECTOR, 0);
> -		else
> +		if (cpu_feature_enabled(X86_FEATURE_FRED)) {
> +			unsigned long edata = 0;
> +
> +			if (cpu_feature_enabled(X86_FEATURE_NMI_SOURCE))
> +				edata = vmx_get_exit_qual(vcpu);
> +			fred_entry_from_kvm(EVENT_TYPE_NMI, NMI_VECTOR, edata);

Eh, I would just east the extra VMREAD if KVM happens to run on a CPU that supports
FRED but not NMI source reporting.  Per spec 7.0, the EXIT_QUALIFICATION is 0 if
NMI source isn't supported, i.e. it's guaranteed to not cause functional problems.

I assume there won't be many (any?) CPUs with FRED but not NMI_SOURCE (and if there
are, why!?!?!).  On the other hand, gating this on NMI_SOURCE means KVM will need
another update if some other event data is ever added for NMIs.  And readers don't
have to hunt through the FRED spec with a fine-toothed comb to suss out that
EXIT_QUALIFICATION == event data, but that KVM is getting cute and only reading
it when it can be non-zero (as far as KVM knows).

As a bonus, the code is less ugly:

		if (cpu_feature_enabled(X86_FEATURE_FRED))
			fred_entry_from_kvm(EVENT_TYPE_NMI, NMI_VECTOR,
					    vmx_get_exit_qual(vcpu));
		else
			vmx_do_nmi_irqoff();

> +		} else {
>  			vmx_do_nmi_irqoff();
> +		}
>  		kvm_after_interrupt(vcpu);
>  	}
>  
> -- 
> 2.25.1
>