Re: [PATCH v2 09/13] MIPS: Add functions for hypervisor call

[Andreas Herrmann <andreas.herrmann@caviumnetworks.com>]

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