linux-kernel - [tip: x86/core] x86,pm: Force out-of-line memcpy()

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Message-ID: <166764458578.4906.12379655891294297691.tip-bot2@tip-bot2>
Date:   Sat, 05 Nov 2022 10:36:25 -0000
From:   "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/core] x86,pm: Force out-of-line memcpy()

The following commit has been merged into the x86/core branch of tip:

Commit-ID:     b32fd8a60f5d855758208c2b5b49cba8087f03c4
Gitweb:        https://git.kernel.org/tip/b32fd8a60f5d855758208c2b5b49cba8087f03c4
Author:        Peter Zijlstra <peterz@...radead.org>
AuthorDate:    Thu, 03 Nov 2022 21:17:03 +01:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Sat, 05 Nov 2022 11:28:02 +01:00

x86,pm: Force out-of-line memcpy()

GCC fancies inlining memcpy(), and because it cannot prove the
destination is page-aligned (it is) it ends up generating atrocious
code like:

 19e:   48 8b 15 00 00 00 00    mov    0x0(%rip),%rdx        # 1a5 <relocate_restore_code+0x25> 1a1: R_X86_64_PC32      core_restore_code-0x4
 1a5:   48 8d 78 08             lea    0x8(%rax),%rdi
 1a9:   48 89 c1                mov    %rax,%rcx
 1ac:   48 c7 c6 00 00 00 00    mov    $0x0,%rsi        1af: R_X86_64_32S       core_restore_code
 1b3:   48 83 e7 f8             and    $0xfffffffffffffff8,%rdi
 1b7:   48 29 f9                sub    %rdi,%rcx
 1ba:   48 89 10                mov    %rdx,(%rax)
 1bd:   48 8b 15 00 00 00 00    mov    0x0(%rip),%rdx        # 1c4 <relocate_restore_code+0x44> 1c0: R_X86_64_PC32      core_restore_code+0xff4
 1c4:   48 29 ce                sub    %rcx,%rsi
 1c7:   81 c1 00 10 00 00       add    $0x1000,%ecx
 1cd:   48 89 90 f8 0f 00 00    mov    %rdx,0xff8(%rax)
 1d4:   c1 e9 03                shr    $0x3,%ecx
 1d7:   f3 48 a5                rep movsq %ds:(%rsi),%es:(%rdi)

Notably the alignment code generates a text reference to
code_restore_code+0xff8, for which objtool raises the objection:

  vmlinux.o: warning: objtool: relocate_restore_code+0x3d: relocation to !ENDBR: next_arg+0x18

Applying some __assume_aligned(PAGE_SIZE) improve code-gen to:

 19e:   48 89 c7                mov    %rax,%rdi
 1a1:   48 c7 c6 00 00 00 00    mov    $0x0,%rsi        1a4: R_X86_64_32S       core_restore_code
 1a8:   b9 00 02 00 00          mov    $0x200,%ecx
 1ad:   f3 48 a5                rep movsq %ds:(%rsi),%es:(%rdi)

And resolve the problem, however, none of this is important code and
a much simpler solution still is to force a memcpy() call:

 1a1:   ba 00 10 00 00          mov    $0x1000,%edx
 1a6:   48 c7 c6 00 00 00 00    mov    $0x0,%rsi        1a9: R_X86_64_32S       core_restore_code
 1ad:   e8 00 00 00 00          call   1b2 <relocate_restore_code+0x32> 1ae: R_X86_64_PLT32     __memcpy-0x4

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/power/hibernate.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/power/hibernate.c b/arch/x86/power/hibernate.c
index e94e005..6f955eb 100644
--- a/arch/x86/power/hibernate.c
+++ b/arch/x86/power/hibernate.c
@@ -159,7 +159,7 @@ int relocate_restore_code(void)
 	if (!relocated_restore_code)
 		return -ENOMEM;
 
-	memcpy((void *)relocated_restore_code, core_restore_code, PAGE_SIZE);
+	__memcpy((void *)relocated_restore_code, core_restore_code, PAGE_SIZE);
 
 	/* Make the page containing the relocated code executable */
 	pgd = (pgd_t *)__va(read_cr3_pa()) +