Re: [patch V4 37/37] x86/smpboot/64: Implement arch_cpuhp_init_parallel_bringup() and enable it

On Fri, May 12, 2023 at 11:07:56PM +0200, Thomas Gleixner wrote:

From: Thomas Gleixner <redacted>

Implement the validation function which tells the core code whether
parallel bringup is possible.

The only condition for now is that the kernel does not run in an encrypted
guest as these will trap the RDMSR via #VC, which cannot be handled at that
point in early startup.

There was an earlier variant for AMD-SEV which used the GHBC protocol for
retrieving the APIC ID via CPUID, but there is no guarantee that the
initial APIC ID in CPUID is the same as the real APIC ID. There is no
enforcement from the secure firmware and the hypervisor can assign APIC IDs
as it sees fit as long as the ACPI/MADT table is consistent with that
assignment.

Unfortunately there is no RDMSR GHCB protocol at the moment, so enabling
AMD-SEV guests for parallel startup needs some more thought.

One option, other than adding said protocol, would be to:

 - use the APICID from CPUID -- with the expectation that it can be
   wrong.
 - (ab)use one of the high bits in cpuid_to_apicid[] as a test-and-set
   trylock. This avoids two CPUs from using the same per-cpu base, if
   CPUID is being malicious. Panic on fail.
 - validate against MSR the moment we can and panic if not matching

The trylock ensures the stacks/percpu state is not used by multiple
CPUs, and should guarantee a coherent state to get far enough along to
be able to do the #VE inducing RDMSR.



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