> On Jun 3, 2014, at 9:40 AM, "Daney, David" <[email protected]>
> wrote:
>
> In cases like this, I always wonder WWPD (What Would Pinski Do)...
>
> Let's get him to opine.
>
> Andrew, the patch in question is:
>
> http://www.linux-mips.org/archives/linux-mips/2014-05/msg00309.html
Yes having two variables with the same register is safe as long as the only
time the live ranges of them overlap is the inline-asm where they are used.
Thanks,
Andrew
>
> Thanks,
> David Daney
>
>> On 06/03/2014 08:03 AM, Andreas Herrmann wrote:
>>> 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
>>
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