Re: [PATCH] x86/paravirt: Use relative reference for original instruction

From: Juergen Gross
Date: Thu Nov 24 2022 - 05:19:46 EST


On 24.11.22 09:51, Hou Wenlong wrote:
Similar to the alternative patching, use relative reference for original
instruction rather than absolute one, which saves 8 bytes for one entry
on x86_64. And it could generate R_X86_64_PC32 relocation instead of

8 bytes saved? I think those are 4 bytes only.

R_X86_64_64 relocation, which also reduces relocation metadata on
relocatable builds.

Signed-off-by: Hou Wenlong <houwenlong.hwl@xxxxxxxxxxxx>
---
arch/x86/include/asm/paravirt.h | 6 +++---
arch/x86/include/asm/paravirt_types.h | 4 ++--
arch/x86/kernel/alternative.c | 8 +++++---
3 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index 2851bc2339d5..2cbe9b64e103 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -735,13 +735,13 @@ extern void default_banner(void);
#else /* __ASSEMBLY__ */
-#define _PVSITE(ptype, ops, word, algn) \
+#define _PVSITE(ptype, ops, algn) \

Would you please drop the algn parameter, too? It isn't needed anymore
as the alignment can be hard coded to be 4 now. This would need to be
adjusted in the _paravirt_alt() macro, too.

771:; \
ops; \
772:; \
.pushsection .parainstructions,"a"; \
.align algn; \
- word 771b; \
+ .long 771b-.; \
.byte ptype; \
.byte 772b-771b; \
_ASM_ALIGN; \
@@ -752,7 +752,7 @@ extern void default_banner(void);
#ifdef CONFIG_PARAVIRT_XXL
#define PARA_PATCH(off) ((off) / 8)
-#define PARA_SITE(ptype, ops) _PVSITE(ptype, ops, .quad, 8)
+#define PARA_SITE(ptype, ops) _PVSITE(ptype, ops, 8)
#define PARA_INDIRECT(addr) *addr(%rip)
#ifdef CONFIG_DEBUG_ENTRY
diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
index 8c1da419260f..19f709cf7df9 100644
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -5,7 +5,7 @@
#ifndef __ASSEMBLY__
/* These all sit in the .parainstructions section to tell us what to patch. */
struct paravirt_patch_site {
- u8 *instr; /* original instructions */
+ s32 instr_offset; /* original instructions */
u8 type; /* type of this instruction */
u8 len; /* length of original instruction */
};
@@ -274,7 +274,7 @@ extern struct paravirt_patch_template pv_ops;
"771:\n\t" insn_string "\n" "772:\n" \
".pushsection .parainstructions,\"a\"\n" \
_ASM_ALIGN "\n" \
- _ASM_PTR " 771b\n" \
+ " .long 771b-.\n" \
" .byte " type "\n" \
" .byte 772b-771b\n" \
_ASM_ALIGN "\n" \
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 111b809f0ac2..6eea563a098d 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1232,20 +1232,22 @@ void __init_or_module apply_paravirt(struct paravirt_patch_site *start,
{
struct paravirt_patch_site *p;
char insn_buff[MAX_PATCH_LEN];
+ u8 *instr;
for (p = start; p < end; p++) {
unsigned int used;
+ instr = (u8 *)&p->instr_offset + p->instr_offset;
BUG_ON(p->len > MAX_PATCH_LEN);
/* prep the buffer with the original instructions */
- memcpy(insn_buff, p->instr, p->len);
- used = paravirt_patch(p->type, insn_buff, (unsigned long)p->instr, p->len);
+ memcpy(insn_buff, instr, p->len);
+ used = paravirt_patch(p->type, insn_buff, (unsigned long)instr, p->len);
BUG_ON(used > p->len);
/* Pad the rest with nops */
add_nops(insn_buff + used, p->len - used);
- text_poke_early(p->instr, insn_buff, p->len);
+ text_poke_early(instr, insn_buff, p->len);
}
}
extern struct paravirt_patch_site __start_parainstructions[],


Juergen

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