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

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Date: Mon, 24 Jun 2024 16:53:59 -0700
From: Sohil Mehta <sohil.mehta@...el.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>, "H. Peter Anvin" <hpa@...or.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


> +	/*
> +	 * 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.
> +	 */

The use of "we" here can be a bit confusing. Writing this in an
imperative mood might make it easier to follow.

> +	if (source_bitmask & BIT(NMI_SOURCE_VEC_UNKNOWN)) {
> +		pr_warn_ratelimited("NMI received with unknown source\n");
> +		return 0;
> +	}
> +

IIUC, bit 0 will be set for out of bounds vectors (>= 16 bit) as well. I
am not sure how realistic that is or if that is even possible to detect?
I am wondering if there should an explicit error message when such a
scenario happens.


> +	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;
> +}
> +