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SubjectRe: [RFC PATCH 03/67] x86/cpu: Move get_builtin_firmware() common code (from microcode only)
On Wed, Nov 25, 2020, Borislav Petkov wrote:
> On Mon, Nov 16, 2020 at 10:25:48AM -0800, isaku.yamahata@intel.com wrote:
> > From: Zhang Chen <chen.zhang@intel.com>
> >
> > Move get_builtin_firmware() to common.c so that it can be used to get
> > non-ucode firmware, e.g. Intel's SEAM modules, even if MICROCODE=n.
>
> What for?
>
> This is used for microcode built in the kernel - a non-common use case.
> Why is your thing built into the kernel and not a normal module object?

The SEAM module needs to be loaded during early boot, it can't be deferred to a
module, at least not without a lot more blood, sweat, and tears.

The SEAM Loader is an ACM that is invoked via GETSEC[EnterACCS], which requires
all APs to be in WFS. SEAM Loader also returns control to the kernel with a
null IDT and NMIs unblocked, i.e. we're toast if there's a pending NMI. And
unlike the run-time SEAMCALLs, boot-time SEAMCALLs do not have a strictly
bounded runtime. Invoking configuration SEAMCALLs after the kernel is fully up
and running could cause instability as IRQ, NMI, and SMI are all blocked in SEAM
mode, e.g. a high priority IRQ/NMI/SMI could be blocked for 50+ usecs (it might
be far more than 50 usecs, I haven't seen real numbers for all SEAMCALLs).

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