x86/asm/entry/32: Explain reloading of registers after __audit_syscall_entry()

Here it is not obvious why we load pt_regs->cx to %esi etc.
Lets improve comments.

Explain that here we combine two things: first, we reload
registers since some of them are clobbered by the C function we
just called; and we also convert 32-bit syscall params to 64-bit
C ABI, because we are going to jump back to syscall dispatch
code.

Move reloading of 6th argument into the macro instead of having
it after each of two macro invocations.

No actual code changes here.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1433876051-26604-2-git-send-email-dvlasenk@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/entry/entry_64_compat.S

@@ -185,12 +185,18 @@ sysexit_from_sys_call:
 	movl	%ebx, %esi		/* 2nd arg: 1st syscall arg */
 	movl	%eax, %edi		/* 1st arg: syscall number */
 	call	__audit_syscall_entry
-	movl	ORIG_RAX(%rsp), %eax	/* reload syscall number */
-	movl	%ebx, %edi		/* reload 1st syscall arg */
-	movl	RCX(%rsp), %esi		/* reload 2nd syscall arg */
-	movl	RDX(%rsp), %edx		/* reload 3rd syscall arg */
-	movl	RSI(%rsp), %ecx		/* reload 4th syscall arg */
-	movl	RDI(%rsp), %r8d		/* reload 5th syscall arg */
+	/*
+	 * We are going to jump back to syscall dispatch.
+	 * Prepare syscall args as required by 64-bit C ABI.
+	 * Clobbered registers are loaded from pt_regs on stack.
+	 */
+	movl	ORIG_RAX(%rsp), %eax	/* syscall number */
+	movl	%ebx, %edi		/* arg1 */
+	movl	RCX(%rsp), %esi		/* arg2 */
+	movl	RDX(%rsp), %edx		/* arg3 */
+	movl	RSI(%rsp), %ecx		/* arg4 */
+	movl	RDI(%rsp), %r8d		/* arg5 */
+	movl	%ebp, %r9d		/* arg6 */
 	.endm
 
 	.macro auditsys_exit exit
@@ -221,7 +227,6 @@ sysexit_from_sys_call:
 
 sysenter_auditsys:
 	auditsys_entry_common
-	movl	%ebp, %r9d		/* reload 6th syscall arg */
 	jmp	sysenter_dispatch
 
 sysexit_audit:
@@ -379,7 +384,6 @@ sysretl_from_sys_call:
 #ifdef CONFIG_AUDITSYSCALL
 cstar_auditsys:
 	auditsys_entry_common
-	movl	%ebp, %r9d		/* reload 6th syscall arg */
 	jmp	cstar_dispatch
 
 sysretl_audit: