Re: [PATCH v2 RESEND] x86/asm: Force native_apic_mem_read to use mov

[Marc Orr]

On Wed, Sep 14, 2022 at 12:13 PM Peter Gonda <pgonda@xxxxxxxxxx> wrote:
>
> On Thu, Sep 8, 2022 at 6:05 PM Adam Dunlap <acdunlap@xxxxxxxxxx> wrote:
> >
> > Previously, when compiled with clang, native_apic_mem_read gets inlined
> > into __xapic_wait_icr_idle and optimized to a testl instruction. When
> > run in a VM with SEV-ES enabled, it attempts to emulate this
> > instruction, but the emulator does not support it. Instead, use inline
> > assembly to force native_apic_mem_read to use the mov instruction which
> > is supported by the emulator.
>
> This seems to be an issue with the SEV-ES in guest #VC handler's
> "emulator" right?
>
> If that's the case I think this should be fixed in the #VC handler
> instead of fixing the code that is failing to be emulated. What if
> there are other places where a testl is used and our tests have not
> caught them.

That was my initial reaction too. But we spoke w/ Tom offline (before
sending this) and my understanding was that we really should be using
MOV for MMIO. I've cc'd Tom so he can speak to this directly though.