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

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Date:   Thu, 24 Nov 2022 19:51:53 +0800
From:   "Hou Wenlong" <houwenlong.hwl@...group.com>
To:     linux-kernel@...r.kernel.org
Cc:     "Hou Wenlong" <houwenlong.hwl@...group.com>,
        "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>,
        "H. Peter Anvin" <hpa@...or.com>,
        "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: [PATCH v2] x86/paravirt: Use relative reference for original instruction

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[],
-- 
2.31.1