On Tue, 2018-01-23 at 21:28 +0100, Borislav Petkov wrote:
> 
> >       flags = (flags & EFLAGS_MASK) | X86_EFLAGS_IF;
> > -     asm("push %[flags]; popf; call *%[fastop]"
> > -         : "=a"(rc) : [fastop]"r"(fop), [flags]"r"(flags));
> > +     asm("push %[flags]; popf; " CALL_NOSPEC
> > +         : "=a"(rc) : [thunk_target]"r"(fop), [flags]"r"(flags));
> 
> Oh, "thunk_target" is magical.

You can use THUNK_TARGET(fop), which will be "rm" on 32-bit and avoids
register starvation in some cases (I don't think the hyperv calls
worked until I did that).