Re: [PATCH v4 04/17] x86/acrn: Introduce hypercall interfaces

Segher Boessenkool
2020-09-30T23:32:29Z

On Wed, Sep 30, 2020 at 12:14:03PM -0700, Nick Desaulniers wrote:
> Do we need register local storage here?

Depends what you want. It looks like you do:

> static inline long bar(unsigned long hcall_id)
> {
> long result;
> asm volatile("movl %1, %%r8d\n\t"
> "vmcall\n\t"
> : "=a" (result)
> : "ir" (hcall_id)
> : );
> return result;
> }

"result" as output from the asm is in %rax, and the compiler will
shuffle that to wherever it needs it as the function return value. That
part will work fine.

But how you are accessing %r8d is not correct, that needs to be a local
register asm (or r8 be made a fixed reg, probably not what you want ;-) )

Segher