Re: [PATCH v2 1/3] xen/ppc: Set up a basic C environment

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.06.2023 03:26, Shawn Anastasio wrote:
> On 6/22/23 5:49 PM, Andrew Cooper wrote:
>> On 22/06/2023 9:57 pm, Shawn Anastasio wrote:
>>> --- /dev/null
>>> +++ b/xen/arch/ppc/setup.c
>>> @@ -0,0 +1,13 @@
>>> +/* SPDX-License-Identifier: GPL-2.0-or-later */
>>> +#include <xen/init.h>
>>> +
>>> +void __init noreturn start_xen(unsigned long r3, unsigned long r4,
>>> +                               unsigned long r5, unsigned long r6,
>>> +                               unsigned long r7)
>>> +{
>>> +    for ( ;; )
>>> +        /* Set current hardware thread to very low priority */
>>> +        asm volatile("or %r31, %r31, %r31");
>>
>> Is there something magic about the OR instruction, or something magic
>> about %r31?
> 
> Using the OR instruction with all three operands equal is of course a
> no-op, but when using certain registers it can have a separate magic
> side effect.
> 
> `or r31,31,31` is one such form that sets the Program Priority Register
> to "very low" priority. Of course here where we don't have SMP going
> there's not much point in using this over the standard side effect-less
> no-op (`or r0,r0,r0` or just `nop`).
> 
> For a table of these magic OR forms, you can see page 836 of the Power
> ISA 3.0B:
> https://wiki.raptorcs.com/w/images/c/cb/PowerISA_public.v3.0B.pdf

I have 3.1 to hand, and it looks like they were dropped from there?
Otherwise I was meaning to say that it's a shame gas doesn't support
these.

Anyway - I think you want to put this behind a macro named after the
pseudo.

Finally, as a nit: Style above is lacking several blanks. One
between the two semicolons, and a total of three in the asm().

Jan