[tip:x86/asm] x86/asm/entry/32: Explain the stub32_clone logic

[tip-bot for Denys Vlasenko]

Commit-ID:  5cdc683b7d8b3341a3d18e0c5498bc1e4f3fb990
Gitweb:     http://git.kernel.org/tip/5cdc683b7d8b3341a3d18e0c5498bc1e4f3fb990
Author:     Denys Vlasenko <dvlasenk@xxxxxxxxxx>
AuthorDate: Wed, 3 Jun 2015 15:58:49 +0200
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Fri, 5 Jun 2015 13:41:27 +0200

x86/asm/entry/32: Explain the stub32_clone logic

The reason for copying of %r8 to %rcx is quite non-obvious.
Add a comment which explains why it is done.

Signed-off-by: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Will Drewry <wad@xxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1433339930-20880-1-git-send-email-dvlasenk@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/entry/ia32entry.S | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/entry/ia32entry.S b/arch/x86/entry/ia32entry.S
index 4bb9f7b..d0c7b28 100644
--- a/arch/x86/entry/ia32entry.S
+++ b/arch/x86/entry/ia32entry.S
@@ -528,6 +528,14 @@ GLOBAL(\label)
 	ALIGN
 GLOBAL(stub32_clone)
 	leaq	sys_clone(%rip), %rax
+	/*
+	 * 32-bit clone API is clone(..., int tls_val, int *child_tidptr).
+	 * 64-bit clone API is clone(..., int *child_tidptr, int tls_val).
+	 * Native 64-bit kernel's sys_clone() implements the latter.
+	 * We need to swap args here. But since tls_val is in fact ignored
+	 * by sys_clone(), we can get away with an assignment
+	 * (arg4 = arg5) instead of a full swap:
+	 */
 	mov	%r8, %rcx
 	jmp	ia32_ptregs_common
 
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