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Date: Thu, 6 Oct 2022 15:15:31 -0500
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Christophe Leroy <christophe.leroy@csgroup.eu>
Subject: Re: [PATCH] powerpc/64s: POWER10 CPU Kconfig build option
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References: <20220923033004.536127-1-npiggin@gmail.com> <4141d141-54a8-0771-ea67-1e457e38f329@csgroup.eu> <CN3KCU5GOMF4.3T533MQ4U2SF0@bobo> <33e5c5e7-a25f-36bf-e7a1-8d6849cd4715@csgroup.eu>
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Hi!

On Thu, Oct 06, 2022 at 06:07:32PM +0000, Christophe Leroy wrote:
> Le 23/09/2022 à 08:23, Nicholas Piggin a écrit :
> > I would rather complete prefixed support in the kernel and use pcrel
> > addressing. Actually even if we don't compile with pcrel or prefixed,
> > there are some instructions and we will probably get more that require
> > prefixed, possible we might want to use them in kernel. Some of it is
> > required to handle user mode instructions too. So I think removing
> > it is premature, but I guess it's up for debate.
> 
> Well ok, in fact I only had code_patching in mind.
> 
> Code patching is only for kernel text. Today code patching is used for 
> things like kprobe, ftrace, etc .... which really do not seems to be 
> prepared for prefixed instructions.
> 
> If you are adding -mno-prefixed, it is worth keeping that code which 
> sometimes gives us some headacke ?

-mpcrel requires -mprefixed.  Using PC relative addressing will be a
significant performance benefit.

> Of course if there are plans to get real prefixed instruction in kernel 
> code anytime soon, lets live with it, in that case the support should 
> get completed. But otherwise I think it would be better to get rid of it 
> for now, and implement it completely when we need it in years.

The future is unstoppable, certainly the near future is :-)

> When I see the following, I'm having hard time believing it would work 
> with prefixed instructions in the kernel text:
> 
> 	typedef u32 kprobe_opcode_t;
> 
> 	struct kprobe {
> 	...
> 		/* Saved opcode (which has been replaced with breakpoint) */
> 		kprobe_opcode_t opcode;
> 
> 
> 	void arch_disarm_kprobe(struct kprobe *p)
> 	{
> 		WARN_ON_ONCE(patch_instruction(p->addr, ppc_inst(p->opcode)));
> 	}

Why would it not work?


Segher