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

[Naveen N Rao]

On Fri, Feb 02, 2024 at 01:02:39PM +1100, Michael Ellerman wrote:
> Segher Boessenkool <segher at 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.

Thanks for clarifying that. I should have done a better job describing 
that in the commit log. I'll update that, add a comment here and send a 
v2.


- Naveen