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

Joerg Roedel
2020-07-15T15:49:03Z

Hi Kees,

as a general note: With SEV-ES the guest kernel will get #VC exceptions
for events that, without SEV-ES, would just cause a #VMEXIT to the
hypervisor.

On Wed, Jul 15, 2020 at 08:26:14AM -0700, Kees Cook wrote:
> On Wed, Jul 15, 2020 at 11:26:38AM +0200, Joerg Roedel wrote:
> > 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.
>
> Oh, hrm, yes, that's true. If other x86 maintainers are comfortable with
> this, then okay. My sense is that changing the early CPU startup paths
> will cause trouble down the line.

The AP startup path does not change for non SEV-ES guests. But under
SEV-ES everything that might cause a #VC exception must be avoided until
the kernel is ready to handle them. With the current patches this
happens when the AP runs in 64bit long-mode and loaded TSS and IDT.
Therefore a slightly different AP boot-path is needed for SEV-ES guests.

> So, going back to the requirements here ... what things in verify_cpu()
> can cause exceptions? AFAICT, cpuid is safely handled (i.e. it is
> detected and only run in a way to avoid exceptions and the MSR
> reads/writes are similarly bound by CPU family/id range checks). I must
> be missing something. :)

It is actually the CPUID instructions that cause #VC exceptions. The
MSRs that are accessed on AMD processors are not intercepted in the
hypervisors this code has been tested on, so these will not cause #VC
exceptions.

Regards,

Joerg