Re: [PATCH v11 3/4] x86/cpu: Do a sanity check on required feature bits

[Borislav Petkov]

On Mon, Mar 23, 2026 at 05:05:48PM +0000, Maciej Wieczor-Retman wrote:
> On 2026-03-23 at 17:31:45 +0100, Borislav Petkov wrote:
> >On Fri, Mar 20, 2026 at 12:50:25PM +0000, Maciej Wieczor-Retman wrote:
> >> From: Maciej Wieczor-Retman <maciej.wieczor-retman@xxxxxxxxx>
> >>
> >> After CPU identification concludes, do a sanity check by comparing the
> >> final x86_capability bitmask with the pre-defined required feature bits.
> >
> >The use being?
> >
> >AFAICT, the required features are:
> >
> >$ cat arch/x86/include/generated/asm/cpufeaturemasks.h
> >
> >...
> >
> >/*
> > * REQUIRED features:
> > *
> > *    FPU MSR PAE CX8 CMOV FXSR XMM XMM2 LM NOPL ALWAYS CPUID
> > */
> >
> >AFAICT, if *any* of those features are not set, the machine will crash'n'burn
> >anyway.
> >
> >So the required features will be "enforced" pretty early :-)
> >
> >Otherwise they're not really required.
> >
> >And besides, what's
> >
> >arch/x86/kernel/verify_cpu.S
> >
> >for if not for that?

> hpa requested to add this patch to the series [1]. As he explained the other modern
> boot stubs don't include the same checks that the Linux kernel has.
> 
> [1] https://lore.kernel.org/all/5fd597a1-46fe-4f46-98ee-d346260c85b5@xxxxxxxxx/

I don't understand.

We call verify_cpu() in startup_64 in kernel proper too.

So if anything's missing, it should be added there instead of adding yet
another let's verify features thing...

-- 
Regards/Gruss,
    Boris.

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