linux-kernel - Re: [PATCH v5 6/9] x86/nmi: Prepare for the new NMI-source vector encoding

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Message-ID: <20250507091722.GC4439@noisy.programming.kicks-ass.net>
Date: Wed, 7 May 2025 11:17:22 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Sohil Mehta <sohil.mehta@...el.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, Xin Li <xin@...or.com>,
	"H . Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...nel.org>,
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	Dave Hansen <dave.hansen@...ux.intel.com>,
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Subject: Re: [PATCH v5 6/9] x86/nmi: Prepare for the new NMI-source vector
 encoding

On Tue, May 06, 2025 at 06:21:42PM -0700, Sohil Mehta wrote:

> +/*
> + * Prepare the delivery mode and vector for the ICR.
> + *
> + * NMI-source vectors have the NMI delivery mode encoded within them to
> + * differentiate them from the IDT vectors. IDT vector 0x2 (NMI_VECTOR)
> + * is treated as an NMI request but without any NMI-source information.
> + */
> +static inline u16 __prepare_ICR_DM_vector(u16 dm_vector)
> +{
> +	u16 vector = dm_vector & APIC_VECTOR_MASK;
> +	u16 dm = dm_vector & APIC_DM_MASK;
> +
> +	if (dm == APIC_DM_NMI) {
> +		/*
> +		 * Pre-FRED, the actual vector is ignored for NMIs, but
> +		 * zero it if NMI-source reporting is not supported to
> +		 * avoid breakage on misbehaving hardware or hypervisors.
> +		 */
> +		if (!cpu_feature_enabled(X86_FEATURE_NMI_SOURCE))
> +			vector = 0;
> +
> +		return dm | vector;
> +	}
> +
> +	if (vector == NMI_VECTOR)
> +		return APIC_DM_NMI;
> +	else

Please drop that else, that's pointless.

> +		return APIC_DM_FIXED | vector;
> +}