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

[Michal Gorlas <michal.gorlas@9elements.com>]

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