Re: [PATCH 41/62] x86/sev-es: Handle MSR events

[Dave Hansen]

On 2/13/20 11:23 PM, Joerg Roedel wrote:
> Yes, investigating this is on the list for future optimizations (besides
> caching CPUID results). My idea is to use alternatives patching for
> this. But the exception handling is needed anyway because #VC
> exceptions happen very early already, basically the first thing after
> setting up a stack is calling verify_cpu(), which uses CPUID.

Ahh, bummer.  How does a guest know that it's running under SEV-ES?
What's the enumeration mechanism if CPUID doesn't "work"?

> The other reason is that things like MMIO and IOIO instructions can't be
> easily patched by alternatives. Those would work with the runtime
> checking you showed above, though.

Is there a reason we can't make a rule that you *must* do MMIO through
an accessor function so we *can* patch them?  I know random drivers
might break the rule, but are SEV-ES guests going to be running random
drivers?  I would think that they mostly if not all want to use virtio.