linux-kernel - Re: [PATCH v2 2/3] firmware: coreboot: loader for Linux-owned SMI handler

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Message-ID: <aF0x-d-uBbMQxrxO@cyber-t14sg4>
Date: Thu, 26 Jun 2025 13:41:45 +0200
From: Michal Gorlas <michal.gorlas@...ements.com>
To: Tzung-Bi Shih <tzungbi@...nel.org>
Cc: Brian Norris <briannorris@...omium.org>,
	Julius Werner <jwerner@...omium.org>, linux-kernel@...r.kernel.org,
	chrome-platform@...ts.linux.dev,
	Marcello Sylvester Bauer <marcello.bauer@...ements.com>
Subject: Re: [PATCH v2 2/3] firmware: coreboot: loader for Linux-owned SMI
 handler

On Wed, Jun 25, 2025 at 02:26:11PM +0200, Michal Gorlas wrote:
> > > +
> > > +	/*
> > > +	 * Gives SMI some time in case it takes longer than expected.
> > > +	 * Only useful on real hardware (tested on RaptorLake), not needed on emulation.
> > > +	 */
> > > +	mdelay(100);
> > 
> > This looks weird. Are there some ways for Linux to be aware of the SMI has
> > completed?
> 
> Not in a straight forward fashion. On Intel SoCs we could read MSR_SMI_COUNT
> [1] before and after sending an SMI, and wait till it increments. I am
> not aware about any unified way that works for AMD SoCs. However, so far
> none of the AMD boards supported by coreboot was tested with MM payload,
> so to make it Intel-only in v3 is not a bad idea.
> 
> [1]: https://elixir.bootlin.com/linux/v6.16-rc3/source/arch/x86/include/asm/msr-index.h#L880

As a follow-up here, making COREBOOT_PAYLOAD_MM dependent on !SMP
resolves the need of acknowledging SMI completion. If SMI takes longer,
Linux is just stalled until SMI handler gives the CPU back to the caller.
I think for this case it could be the way, LinuxBoot is by default
compiled without SMP support anyways when used as coreboot's payload [1],
[2].

[1]: https://github.com/coreboot/coreboot/blob/main/payloads/external/LinuxBoot/x86_64/defconfig
[2]: https://github.com/coreboot/coreboot/blob/main/payloads/external/LinuxBoot/i386/defconfig