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Date:   Sun, 27 Nov 2022 09:24:34 -0800
From:   "H. Peter Anvin" <hpa@zytor.com>
To:     Hou Wenlong <houwenlong.hwl@antgroup.com>,
        linux-kernel@vger.kernel.org
CC:     Juergen Gross <jgross@suse.com>,
        "Srivatsa S. Bhat (VMware)" <srivatsa@csail.mit.edu>,
        Alexey Makhalov <amakhalov@vmware.com>,
        VMware PV-Drivers Reviewers <pv-drivers@vmware.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        Kees Cook <keescook@chromium.org>, Song Liu <song@kernel.org>,
        Nadav Amit <namit@vmware.com>,
        virtualization@lists.linux-foundation.org
Subject: =?US-ASCII?Q?Re=3A_=5BPATCH_v2=5D_x86/paravirt=3A_Use_relat?= =?US-ASCII?Q?ive_reference_for_original_instruction?=
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On November 24, 2022 3:51:53 AM PST, Hou Wenlong <houwenlong=2Ehwl@antgroup=
=2Ecom> 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=2E  And it could generate R_X86_64_PC32 relocation instead of
>R_X86_64_64 relocation, which also reduces relocation metadata on
>relocatable builds=2E And the alignment could be hard coded to be 4 now=
=2E
>
>Signed-off-by: Hou Wenlong <houwenlong=2Ehwl@antgroup=2Ecom>
>---
> arch/x86/include/asm/paravirt=2Eh       | 10 +++++-----
> arch/x86/include/asm/paravirt_types=2Eh |  8 ++++----
> arch/x86/kernel/alternative=2Ec         |  8 +++++---
> 3 files changed, 14 insertions(+), 12 deletions(-)
>
>diff --git a/arch/x86/include/asm/paravirt=2Eh b/arch/x86/include/asm/par=
avirt=2Eh
>index 2851bc2339d5=2E=2Ee56065ea73f2 100644
>--- a/arch/x86/include/asm/paravirt=2Eh
>+++ b/arch/x86/include/asm/paravirt=2Eh
>@@ -735,16 +735,16 @@ extern void default_banner(void);
>=20
> #else  /* __ASSEMBLY__ */
>=20
>-#define _PVSITE(ptype, ops, word, algn)		\
>+#define _PVSITE(ptype, ops)			\
> 771:;						\
> 	ops;					\
> 772:;						\
> 	=2Epushsection =2Eparainstructions,"a";	\
>-	 =2Ealign	algn;				\
>-	 word 771b;				\
>+	 =2Ealign	4;				\
>+	 =2Elong 771b-=2E;				\
> 	 =2Ebyte ptype;				\
> 	 =2Ebyte 772b-771b;			\
>-	 _ASM_ALIGN;				\
>+	 =2Ealign 4;				\
> 	=2Epopsection
>=20
>=20
>@@ -752,7 +752,7 @@ extern void default_banner(void);
> #ifdef CONFIG_PARAVIRT_XXL
>=20
> #define PARA_PATCH(off)		((off) / 8)
>-#define PARA_SITE(ptype, ops)	_PVSITE(ptype, ops, =2Equad, 8)
>+#define PARA_SITE(ptype, ops)	_PVSITE(ptype, ops)
> #define PARA_INDIRECT(addr)	*addr(%rip)
>=20
> #ifdef CONFIG_DEBUG_ENTRY
>diff --git a/arch/x86/include/asm/paravirt_types=2Eh b/arch/x86/include/a=
sm/paravirt_types=2Eh
>index 8c1da419260f=2E=2E68952ae07a3f 100644
>--- a/arch/x86/include/asm/paravirt_types=2Eh
>+++ b/arch/x86/include/asm/paravirt_types=2Eh
>@@ -5,7 +5,7 @@
> #ifndef __ASSEMBLY__
> /* These all sit in the =2Eparainstructions section to tell us what to p=
atch=2E */
> 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"		\
> 	"=2Epushsection =2Eparainstructions,\"a\"\n"	\
>-	_ASM_ALIGN "\n"					\
>-	_ASM_PTR " 771b\n"				\
>+	"  =2Ealign 4\n"					\
>+	"  =2Elong 771b-=2E\n"				\
> 	"  =2Ebyte " type "\n"				\
> 	"  =2Ebyte 772b-771b\n"				\
>-	_ASM_ALIGN "\n"					\
>+	"  =2Ealign 4\n"					\
> 	"=2Epopsection\n"
>=20
> /* Generate patchable code, with the default asm parameters=2E */
>diff --git a/arch/x86/kernel/alternative=2Ec b/arch/x86/kernel/alternativ=
e=2Ec
>index 111b809f0ac2=2E=2E6eea563a098d 100644
>--- a/arch/x86/kernel/alternative=2Ec
>+++ b/arch/x86/kernel/alternative=2Ec
>@@ -1232,20 +1232,22 @@ void __init_or_module apply_paravirt(struct parav=
irt_patch_site *start,
> {
> 	struct paravirt_patch_site *p;
> 	char insn_buff[MAX_PATCH_LEN];
>+	u8 *instr;
>=20
> 	for (p =3D start; p < end; p++) {
> 		unsigned int used;
>=20
>+		instr =3D (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 =3D paravirt_patch(p->type, insn_buff, (unsigned long)p->instr, p=
->len);
>+		memcpy(insn_buff, instr, p->len);
>+		used =3D paravirt_patch(p->type, insn_buff, (unsigned long)instr, p->l=
en);
>=20
> 		BUG_ON(used > p->len);
>=20
> 		/* 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?