Re: [PATCH v4 70/75] x86/head/64: Don't call verify_cpu() on starting APs

[Joerg Roedel]

Hi Kees,

thanks for your reviews!

On Tue, Jul 14, 2020 at 06:40:30PM -0700, Kees Cook wrote:
> Eek, no. MSR_IA32_MISC_ENABLE_XD_DISABLE needs to be cleared very early
> during CPU startup; this can't just be skipped.

That MSR is Intel-only, right? The boot-path installed here is only used
for SEV-ES guests, running on AMD systems, so this MSR is not even
accessed during boot on those VMs.

The alternative is to set up exception handling prior to calling
verify_cpu, including segments, stack and IDT. Given that verify_cpu()
does not add much value to SEV-ES guests, I'd like to avoid adding this
complexity.

> Also, is UNWIND_HINT_EMPTY needed for the new target?

Yes, I think it is, will add it in the next version.

Regards,

	Joerg