Re: [PATCH v2 1/2] x86/cpu: Disable CR pinning during CPU bringup

[Borislav Petkov]

On Mon, Mar 09, 2026 at 08:38:10AM -0700, Dave Hansen wrote:
> On 3/9/26 06:46, Borislav Petkov wrote:
> > My SNP guest stops booting with this right:
> 
> Could you dump out CR4 at wakeup_cpu_via_vmgexit() before and after this
> patch? Right here:
> 
>         /* CR4 should maintain the MCE value */
>         cr4 = native_read_cr4() & X86_CR4_MCE;
> 
> It's got to be some delta there.

Looks the same to me:

before:      31  SEV: wakeup_cpu_via_vmgexit: CR4: 0x3506f0

That's 31 CPUs - no BSP with the CR4 value above.

after: [    3.354326] SEV: wakeup_cpu_via_vmgexit: CR4: 0x3506f0

That stops after CPU1, i.e., the first AP. But the CR4 value is the same.

> The other possibility is that some CR4 bit becomes no longer pinned when
> the CPU comes up, and the *pinning* was what caused the secondary CPU's
> CR4 bit to get set, not its actual initialization.
> 
> Basically, the secondary boot code didn't explicitly set a bit and
> counted on the pinning code to do it instead.
> 
> It's probably exacerbated by the "novel" way that SEV-SNP CPUs get
> brought up and all the assembly that *only* runs there.

I guess I can start commenting out things to see what happens. Hmmm...

-- 
Regards/Gruss,
    Boris.

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