On Tue, Jun 03, 2014 at 10:30:31AM +0200, Ralf Baechle wrote:
> On Wed, May 28, 2014 at 11:52:12PM +0200, Andreas Herrmann wrote:
>
> > +/*
> > + * Hypercalls for KVM.
> > + *
> > + * Hypercall number is passed in v0.
> > + * Return value will be placed in v0.
> > + * Up to 3 arguments are passed in a0, a1, and a2.
> > + */
> > +static inline unsigned long kvm_hypercall0(unsigned long num)
> > +{
> > + register unsigned long n asm("v0");
> > + register unsigned long r asm("v0");
>
> Btw, is it safe to put two variables in the same register?
I think it's safe.
If we would have a matching constraint letter (say "v" for register v0) the
asm should translate to
__asm__ __volatile__(
KVM_HYPERCALL
: "=v" (n) : "v" (r) : "memory"
);
which isn't unusual on other archs. (Or maybe I am just biased from
x86 ... or missed something else.)
> The syscall wrappers that used to be in <asm/unistd.h> were occasionally
> hitting problems which eventually forced me to stop forcing variables
> into particular registers instead using a MOVE instruction to shove
> each variable into the right place.
>
> Of course they were being used from non-PIC and PIC code, kernel and userland
> so GCC had a much better chance to do evil than in the hypercall wrapper
> case - but it made me paranoid ...
Andreas
