Re: [Xen-devel] [PATCH v3 2/5] x86/hyperv: provide Hyper-V hypercall functions

[Wei Liu <wl@xxxxxxx>]

On Sun, Jan 05, 2020 at 07:08:28PM +0000, Andrew Cooper wrote:
> 
> > +static inline uint64_t hv_do_hypercall(uint64_t control, paddr_t input, 
> > paddr_t output)
> > +{
> > +    uint64_t status;
> > +
> > +    asm volatile ("mov %[output], %%r8\n"
> > +                  "call hv_hypercall_page"
> > +                  : "=a" (status), "+c" (control),
> > +                    "+d" (input) ASM_CALL_CONSTRAINT
> > +                  : [output] "rm" (output)
> > +                  : "cc", "memory", "r8", "r9", "r10", "r11");
> 
> I think you want:
> 
> register unsigned long r8 asm("r8") = output;
> 
> and "+r" (r8) as an output constraint.
> 
> In particular, that doesn't force the compiler to put output into a
> register other than r8 (or worse, spill it to the stack) to have the
> opaque blob of asm move it back into r8.  What it will do in practice is
> cause the compiler to construct output directly in r8.
> 
> As for the other clobbers, I can't find anything at all in the spec
> which even mentions those registers.  There will be a decent improvement
> to code generation if we don't force them to be spilled around a hypercall.

This is actually from Windows 2012 R2's TLFS.

Current version of Hyper-V doesn't clobber those registers anymore. I
think just putting "memory" there should be fine.

Wei.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel