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SubjectRe: [PATCH] x86/paravirt: Use relative reference for original instruction
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@antgroup.com>
> ---
> 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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