linux-kernel - Re: [PATCH] loongarch: Only select HAVE_OBJTOOL and allow ORC unwinder if the inline assembler supports R_LARCH_{32,64}

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Date: Thu, 06 Jun 2024 03:05:41 +0800
From: Xi Ruoyao <xry111@...111.site>
To: Jinyang He <hejinyang@...ngson.cn>, Nathan Chancellor
 <nathan@...nel.org>,  Peter Zijlstra <peterz@...radead.org>
Cc: Huacai Chen <chenhuacai@...nel.org>, WANG Xuerui <kernel@...0n.name>, 
 Tiezhu Yang <yangtiezhu@...ngson.cn>, Nick Desaulniers
 <ndesaulniers@...gle.com>, Bill Wendling <morbo@...gle.com>, Justin Stitt
 <justinstitt@...gle.com>, Youling Tang <tangyouling@...inos.cn>,
 loongarch@...ts.linux.dev,  linux-kernel@...r.kernel.org,
 llvm@...ts.linux.dev, mengqinggang@...ngson.cn,  cailulu@...ngson.cn,
 wanglei@...ngson.cn, luweining@...ngson.cn, Yujie Liu
 <yujie.liu@...el.com>, Heng Qi <hengqi@...ux.alibaba.com>, Tejun Heo
 <tj@...nel.org>
Subject: Re: [PATCH] loongarch: Only select HAVE_OBJTOOL and allow ORC
 unwinder if the inline assembler supports R_LARCH_{32,64}_PCREL

On Wed, 2024-06-05 at 23:47 +0800, Jinyang He wrote:
> In our 419 repo this func has been renamed arch_update_insn_state (, now it
> should be arch_update_cfi_state) and give some actions to deal with the
> frame pointer case. If we support it we may deal with some case but for 
> clang...
> 
> .global test
> .type test,@function
> test:
> 
> addi.d  $sp,$sp,-448
> st.d    $ra,$sp,440
> st.d    $fp,$sp,432
> addi.d  $fp,$sp,448
> 
> # do something  <- not change $sp

This is simplified.  In the real code $sp is changed, something like:

bstrins.d $sp, $zero, 5, 0

$fp is needed to allow restoring $sp after realigning $sp for some local
over-aligned variables, as demonstrated by this example:

struct x { _Alignas(64) char buf[128]; };

void f(struct x *p);
void g()
{
	struct x x = { .buf = "1145141919810" };
	f(&x);
}

GCC does not align $sp, it produces the aligned address for x from an
unaligned $sp instead:

addi.d $a0, $sp, 63
srli.d $a0, $a0, 6
slli.d $a0, $a0, 6

thus there's no need to use $fp.

/* snip */

> <- restore regs from non-cfa ?
>          ld.d    $ra, $sp, 8                     # 8-byte Folded Reload  

/* snip */

> Maybe Clang have anything wrong?

I don't think we must restore regs based on $fp just because CFA is
based on $fp.  The .cfi directives only provides *one* possible way to
restore the regs.  This way is convenient to the unwinder, but not
necessarily convenient to the CPU thus the real instruction sequence can
use a different way.  They only need to be "equivalent", not necessarily
"exactly same."

Also note that .cfi_* directives are completely irrelevant for objtool.
THe objtool synthesizes the ORC unwind info from the machine
instructions, not the DWARF unwind info coded with .cfi_*.

The entire point of ORC is avoiding DWARF.  It's even named ORC because
ORC and DWARF are sworn enemies in some tales.

-- 
Xi Ruoyao <xry111@...111.site>
School of Aerospace Science and Technology, Xidian University