linux-kernel - Re: [RFC PATCH 15/26] x86/alternatives: Non-emulated text poking

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Message-ID: <20200408111322.GU20713@hirez.programming.kicks-ass.net>
Date:   Wed, 8 Apr 2020 13:13:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ankur Arora <ankur.a.arora@...cle.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, hpa@...or.com,
        jpoimboe@...hat.com, namit@...are.com, mhiramat@...nel.org,
        jgross@...e.com, bp@...en8.de, vkuznets@...hat.com,
        pbonzini@...hat.com, boris.ostrovsky@...cle.com,
        mihai.carabas@...cle.com, kvm@...r.kernel.org,
        xen-devel@...ts.xenproject.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [RFC PATCH 15/26] x86/alternatives: Non-emulated text poking

On Tue, Apr 07, 2020 at 10:03:12PM -0700, Ankur Arora wrote:
> +static void __maybe_unused sync_one(void)
> +{
> +	/*
> +	 * We might be executing in NMI context, and so cannot use
> +	 * IRET as a synchronizing instruction.
> +	 *
> +	 * We could use native_write_cr2() but that is not guaranteed
> +	 * to work on Xen-PV -- it is emulated by Xen and might not
> +	 * execute an iret (or similar synchronizing instruction)
> +	 * internally.
> +	 *
> +	 * cpuid() would trap as well. Unclear if that's a solution
> +	 * either.
> +	 */
> +	if (in_nmi())
> +		cpuid_eax(1);
> +	else
> +		sync_core();
> +}

That's not thinking staight; what do you think the INT3 does when it
happens inside an NMI ?