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

[Peter Zijlstra]

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 ?