linux-kernel - Re: [PATCH v2] x86/paravirt: Use relative reference for original instruction

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Date:   Sun, 27 Nov 2022 09:24:34 -0800
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Hou Wenlong <houwenlong.hwl@...group.com>,
        linux-kernel@...r.kernel.org
CC:     Juergen Gross <jgross@...e.com>,
        "Srivatsa S. Bhat (VMware)" <srivatsa@...il.mit.edu>,
        Alexey Makhalov <amakhalov@...are.com>,
        VMware PV-Drivers Reviewers <pv-drivers@...are.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Kees Cook <keescook@...omium.org>, Song Liu <song@...nel.org>,
        Nadav Amit <namit@...are.com>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v2] x86/paravirt: Use relative reference for original instruction

On November 24, 2022 3:51:53 AM PST, Hou Wenlong <houwenlong.hwl@...group.com> 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
>R_X86_64_64 relocation, which also reduces relocation metadata on
>relocatable builds. And the alignment could be hard coded to be 4 now.
>
>Signed-off-by: Hou Wenlong <houwenlong.hwl@...group.com>
>---
> arch/x86/include/asm/paravirt.h       | 10 +++++-----
> arch/x86/include/asm/paravirt_types.h |  8 ++++----
> arch/x86/kernel/alternative.c         |  8 +++++---
> 3 files changed, 14 insertions(+), 12 deletions(-)
>
>diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
>index 2851bc2339d5..e56065ea73f2 100644
>--- a/arch/x86/include/asm/paravirt.h
>+++ b/arch/x86/include/asm/paravirt.h
>@@ -735,16 +735,16 @@ extern void default_banner(void);
> 
> #else  /* __ASSEMBLY__ */
> 
>-#define _PVSITE(ptype, ops, word, algn)		\
>+#define _PVSITE(ptype, ops)			\
> 771:;						\
> 	ops;					\
> 772:;						\
> 	.pushsection .parainstructions,"a";	\
>-	 .align	algn;				\
>-	 word 771b;				\
>+	 .align	4;				\
>+	 .long 771b-.;				\
> 	 .byte ptype;				\
> 	 .byte 772b-771b;			\
>-	 _ASM_ALIGN;				\
>+	 .align 4;				\
> 	.popsection
> 
> 
>@@ -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)
> #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..68952ae07a3f 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 */
> };
>@@ -273,11 +273,11 @@ extern struct paravirt_patch_template pv_ops;
> #define _paravirt_alt(insn_string, type)		\
> 	"771:\n\t" insn_string "\n" "772:\n"		\
> 	".pushsection .parainstructions,\"a\"\n"	\
>-	_ASM_ALIGN "\n"					\
>-	_ASM_PTR " 771b\n"				\
>+	"  .align 4\n"					\
>+	"  .long 771b-.\n"				\
> 	"  .byte " type "\n"				\
> 	"  .byte 772b-771b\n"				\
>-	_ASM_ALIGN "\n"					\
>+	"  .align 4\n"					\
> 	".popsection\n"
> 
> /* Generate patchable code, with the default asm parameters. */
>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[],

Any reason that you couldn't use the same patching code?