linux-kernel - Re: [PATCH v2 4/6] x86/irq: Process nmi sources in NMI handler

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Date: Tue, 11 Jun 2024 11:41:07 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Jacob Pan <jacob.jun.pan@...ux.intel.com>, X86 Kernel <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...el.com>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>, linux-perf-users@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>
Cc: Andi Kleen <andi.kleen@...el.com>, Xin Li <xin3.li@...el.com>
Subject: Re: [PATCH v2 4/6] x86/irq: Process nmi sources in NMI handler

On 6/11/24 09:54, Jacob Pan wrote:
> +
> +	source_bitmask = fred_event_data(regs);
> +	if (!source_bitmask) {
> +		pr_warn_ratelimited("NMI without source information! Disable source reporting.\n");
> +		setup_clear_cpu_cap(X86_FEATURE_NMI_SOURCE);
> +		return 0;
> +	}

Is setup_clear_cpu_cap() even meaningful here?

> +
> +	/*
> +	 * Per NMI source specification, there is no guarantee that a valid
> +	 * NMI vector is always delivered, even when the source specified
> +	 * one. It is software's responsibility to check all available NMI
> +	 * sources when bit 0 is set in the NMI source bitmap. i.e. we have
> +	 * to call every handler as if we have no NMI source.
> +	 * On the other hand, if we do get non-zero vectors, we know exactly
> +	 * what the sources are. So we only call the handlers with the bit set.
> +	 */
> +	if (source_bitmask & BIT(NMI_SOURCE_VEC_UNKNOWN)) {
> +		pr_warn_ratelimited("NMI received with unknown source\n");
> +		return 0;
> +	}
> +

You can still dispatch the known NMI handlers early before doing the 
polling.

> +	rcu_read_lock();
> +	/* Bit 0 is for unknown NMI sources, skip it. */
> +	for_each_set_bit_from(vec, &source_bitmask, NR_NMI_SOURCE_VECTORS) {
> +		a = rcu_dereference(nmiaction_src_table[vec]);
> +		if (!a) {
> +			pr_warn_ratelimited("NMI received %d no handler", vec);
> +			continue;
> +		}
> +		handled += do_handle_nmi(a, regs, type);
> +	}
> +	rcu_read_unlock();
> +	return handled;
> +}
> +

That would mean that you would also need to return a bitmask of which 
source vectors need to be handled with polling.

	-hpa