Re: [PATCH] powerpc/64: Set LR to a non-NULL value in task pt_regs on scv entry

[Michael Ellerman <mpe@ellerman.id.au>]

Segher Boessenkool <segher@kernel.crashing.org> writes:
> Hi!
>
> On Thu, Jan 25, 2024 at 05:12:28PM +0530, Naveen N Rao wrote:
>> diff --git a/arch/powerpc/kernel/interrupt_64.S b/arch/powerpc/kernel/interrupt_64.S
>> index bd863702d812..5cf3758a19d3 100644
>> --- a/arch/powerpc/kernel/interrupt_64.S
>> +++ b/arch/powerpc/kernel/interrupt_64.S
>> @@ -53,6 +53,7 @@ _ASM_NOKPROBE_SYMBOL(system_call_vectored_\name)
>>  	ld	r1,PACAKSAVE(r13)
>>  	std	r10,0(r1)
>>  	std	r11,_NIP(r1)
>> +	std	r11,_LINK(r1)
>
> Please add a comment here then, saying what the store is for?

Yeah a comment would be good. 

Also the r11 value comes from LR, so it's not that we're storing the NIP
value into the LR slot, rather the value we store in NIP is from LR, see:

EXC_VIRT_BEGIN(system_call_vectored, 0x3000, 0x1000)
	/* SCV 0 */
	mr	r9,r13
	GET_PACA(r13)
	mflr	r11
        ...
	b	system_call_vectored_common

That's slightly pedantic, but I think it answers the question of why
it's OK to use the same value for NIP & LR, or why we don't have to do
mflr in system_call_vectored_common to get the actual LR value.

cheers