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Date:   Sun, 27 Nov 2022 19:34:39 -0800
From:   "H. Peter Anvin" <hpa@zytor.com>
To:     Hou Wenlong <houwenlong.hwl@antgroup.com>
CC:     linux-kernel@vger.kernel.org, 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 27, 2022 7:03:20 PM PST, Hou Wenlong <houwenlong=2Ehwl@antgroup=
=2Ecom> wrote:
>On Sun, Nov 27, 2022 at 09:24:34AM -0800, H=2E Peter Anvin wrote:
>> On November 24, 2022 3:51:53 AM PST, Hou Wenlong <houwenlong=2Ehwl@antg=
roup=2Ecom> wrote:
>> >Similar to the alternative patching, use relative reference for origin=
al
>> >instruction rather than absolute one, which saves 8 bytes for one entr=
y
>> >on x86_64=2E  And it could generate R_X86_64_PC32 relocation instead o=
f
>> >R_X86_64_64 relocation, which also reduces relocation metadata on
>> >relocatable builds=2E And the alignment could be hard coded to be 4 no=
w=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/=
paravirt=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/includ=
e/asm/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 t=
o patch=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/alterna=
tive=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 pa=
ravirt_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=
->len);
>> >=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[],
>>=20
>> Any reason that you couldn't use the same patching code?
>
>Sorry, what do you mean using the same patching code? Do you
>mean that share some code between apply_alternatives() and
>apply_paravirt()?

Yes=2E Abstract the facility rather than duplicate=2E