[PATCH] powerpc: don't try to copy ppc for task with NULL pt_regs

[Michael Ellerman]

"Nicholas Piggin" <npiggin at gmail.com> writes:
> On Mon Mar 27, 2023 at 8:26 PM AEST, Christophe Leroy wrote:
...
>>
>> Now that thread.regs doesn't change anymore at each interrupt, it would 
>> probably be worth dropping it and falling back to task_pt_regs() as 
>> defined on most architecture.
>> Knowing whether a thread is a kernel or user thread can for instance be 
>> known with PF_KTHREAD flag, so no need of thread.regs for that.
>
> That would be nice if we can define regs that way, I agree. We should
> look into doing that.

Yeah it's on the long-list of things that need cleaning up.

I think there's some complication in working out which sites are OK to
use/give-out the value in pt_regs that's potentially a dummy value, vs
cases that actually want to check PF_KTHREAD and do something different.
But that's just my hunch I haven't looked through all the sites.

The thread.regs = NULL for kernel threads goes back to arch/ppc, about
2002 by the looks:

  https://github.com/mpe/linux-fullhistory/commit/2a8e186c384c0c911f91cd12367658eabdc820d8#diff-939b705cff722ee75595fad30d56bb1175dfdce49a69adb4d5656f354be076c6

There's no change log of course :)

Still maybe it doesn't matter why it was originally done that way, if we
can do it differently now.

cheers