linux-kernel - Re: [PATCH v4 2/5] LoongArch: Use la.pcrel instead of la.abs for exception handlers

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Date:   Thu, 16 Feb 2023 19:18:57 +0800
From:   Youling Tang <tangyouling@...ngson.cn>
To:     Xi Ruoyao <xry111@...111.site>, Jinyang He <hejinyang@...ngson.cn>,
        Huacai Chen <chenhuacai@...nel.org>
Cc:     Xuerui Wang <kernel@...0n.name>, loongarch@...ts.linux.dev,
        linux-kernel@...r.kernel.org, Xuefeng Li <lixuefeng@...ngson.cn>,
        Jianmin lv <lvjianmin@...ngson.cn>,
        Tiezhu Yang <yangtiezhu@...ngson.cn>
Subject: Re: [PATCH v4 2/5] LoongArch: Use la.pcrel instead of la.abs for
 exception handlers


On 02/16/2023 04:03 PM, Youling Tang wrote:
>
>
> On 02/16/2023 03:10 PM, Xi Ruoyao wrote:
>> On Thu, 2023-02-16 at 14:59 +0800, Jinyang He wrote:
>>> +.macro la.abs reg, sym
>>> +766:
>>> +    nop
>>> +    nop
>>> +    nop
>>> +    nop
>>
>> In the "formal" version we can code
>>
>> lu12i.w        reg, 0
>> ori        reg, reg, 0
>> lu32i.d        reg, 0
>> lu52i.d        reg, reg, 0
>>
>> here.  Then we only need to fixup the immediate slot so we can avoid
>> using parse_r.
>>
>>
>>> +    .pushsection ".laabs", "aw", %progbits
>>> +768:
>>> +    .word 768b-766b
>>> +    parse_r regno, \reg
>>> +    .word regno
>>> +    .dword \sym
>>> +    .popsection
>>> +.endm
>
> I will try to modify a version for testing, using the following
> definition, when the RELOCATABLE is turned on, the "la.abs macro" is
> used, otherwise the "la.abs pseudo instruction" is still used as before.
>
> #ifdef CONFIG_RELOCATABLE
> .macro la.abs reg, sym
> lu12i.w        reg, 0
> ori        reg, reg, 0
> lu32i.d        reg, 0
> lu52i.d        reg, reg, 0
> .endm
> #endif

On the basis of the v4 patch set, remove patch2, and then add the 
following patches, and the test is successful on qemu.

If this method is more acceptable to everyone, I will send v5.

diff --git a/arch/loongarch/include/asm/asmmacro.h 
b/arch/loongarch/include/asm/asmmacro.h
index 328bb956f241..adb04ae6b208 100644
--- a/arch/loongarch/include/asm/asmmacro.h
+++ b/arch/loongarch/include/asm/asmmacro.h
@@ -667,4 +667,19 @@
         nor     \dst, \src, zero
  .endm

+#ifdef CONFIG_RELOCATABLE
+.macro la.abs reg, sym
+766:
+       lu12i.w \reg, 0
+       ori     \reg, \reg, 0
+       lu32i.d \reg, 0
+       lu52i.d \reg, \reg, 0
+       .pushsection ".laabs", "aw", %progbits
+768:
+       .dword 768b-766b
+       .dword \sym
+       .popsection
+.endm
+#endif
+
  #endif /* _ASM_ASMMACRO_H */
diff --git a/arch/loongarch/kernel/relocate.c 
b/arch/loongarch/kernel/relocate.c
index 7d19cc0d2185..7ad327a554f9 100644
--- a/arch/loongarch/kernel/relocate.c
+++ b/arch/loongarch/kernel/relocate.c
@@ -12,6 +12,7 @@
  #include <linux/start_kernel.h>
  #include <asm/bootinfo.h>
  #include <asm/early_ioremap.h>
+#include <asm/inst.h>
  #include <asm/sections.h>

  #define RELOCATED(x) ((void *)((long)x + reloc_offset))
@@ -45,6 +46,32 @@ static inline __init void relocate_relative(void)
         }
  }

+static inline void __init relocate_laabs(long offset)
+{
+       extern void *__laabs_begin;
+       extern void *__laabs_end;
+       struct laabs {
+               long offset;
+               long symvalue;
+       } *p;
+
+       for (p = (void *)&__laabs_begin; (void *)p < (void 
*)&__laabs_end; p++) {
+               long v = p->symvalue + reloc_offset;
+               union loongarch_instruction *insn = (void *)p - 
p->offset + offset;
+               u32 lu12iw, ori, lu32id, lu52id;
+
+               lu12iw = (v >> 12) & 0xfffff;
+               ori = v & 0xfff;
+               lu32id = (v >> 32) & 0xfffff;
+               lu52id = v >> 52;
+
+               insn[0].reg1i20_format.immediate = lu12iw;
+               insn[1].reg2i12_format.immediate = ori;
+               insn[2].reg1i20_format.immediate = lu32id;
+               insn[3].reg2i12_format.immediate = lu52id;
+    }
+}
+
  #ifdef CONFIG_RANDOMIZE_BASE
  static inline __init unsigned long rotate_xor(unsigned long hash,
                                               const void *area, size_t 
size)
@@ -168,8 +195,10 @@ void *__init do_kaslr(void)
                 update_reloc_offset(&reloc_offset, offset);
         }

-       if (reloc_offset)
+       if (reloc_offset) {
                 relocate_relative();
+               relocate_laabs(offset);
+       }

         return kernel_entry;
  }
@@ -181,6 +210,8 @@ void __init relocate_kernel(void)

         if (reloc_offset)
                 relocate_relative();
+
+       relocate_laabs(0);
  }

  /*
diff --git a/arch/loongarch/kernel/vmlinux.lds.S 
b/arch/loongarch/kernel/vmlinux.lds.S
index aec0b6567d24..0e58c68bf427 100644
--- a/arch/loongarch/kernel/vmlinux.lds.S
+++ b/arch/loongarch/kernel/vmlinux.lds.S
@@ -66,6 +66,13 @@ SECTIONS
                 __alt_instructions_end = .;
         }

+       . = ALIGN(8);
+       .laabs : AT(ADDR(.laabs) - LOAD_OFFSET) {
+               __laabs_begin = .;
+               *(.laabs)
+               __laabs_end = .;
+       }
+
         .got : ALIGN(16) { *(.got) }


Youling.