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

[Shawn Anastasio <sanastasio@xxxxxxxxxxxxxxxxxxxxx>]

On 6/23/23 1:34 AM, Jan Beulich wrote:
> 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.

No, they're still present in ISA 3.1B. See page 1084, Book II Chapter
3 Section 2.

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

Sure, that makes sense to me.

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

Just to be sure, would the following be correct?

    for ( ; ; )
        /* Set current hardware thread to very low priority */
        asm volatile ( "or %r31, %r31, %r31" );

Not including the refactor of that instruction to a macro, of course.

> Jan

Thanks,
Shawn