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authorAlbert D. Cahalan <acahalan@cs.uml.edu>2006-02-14 13:53:20 -0800
committerLinus Torvalds <torvalds@g5.osdl.org>2006-02-14 16:09:35 -0800
commit581141cb4b10ebd865dcb7b80f5e712e2def5408 (patch)
treecd9c6e4ac3c53196586f1246439e417410b90a4f /arch
parent28baebae73c3ea8b75c7cae225a7db817ab825a9 (diff)
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[PATCH] x86: document sysenter path
This path isn't obvious. It looks as if the kernel will be taking three args from the user stack, but it only takes one from there. Signed-off-by: Albert Cahalan <acahalan@gmail.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Diffstat (limited to 'arch')
-rw-r--r--arch/i386/kernel/vsyscall-sysenter.S15
1 files changed, 15 insertions, 0 deletions
diff --git a/arch/i386/kernel/vsyscall-sysenter.S b/arch/i386/kernel/vsyscall-sysenter.S
index 4daefb2..76b7281 100644
--- a/arch/i386/kernel/vsyscall-sysenter.S
+++ b/arch/i386/kernel/vsyscall-sysenter.S
@@ -7,6 +7,21 @@
* for details.
*/
+/*
+ * The caller puts arg2 in %ecx, which gets pushed. The kernel will use
+ * %ecx itself for arg2. The pushing is because the sysexit instruction
+ * (found in entry.S) requires that we clobber %ecx with the desired %esp.
+ * User code might expect that %ecx is unclobbered though, as it would be
+ * for returning via the iret instruction, so we must push and pop.
+ *
+ * The caller puts arg3 in %edx, which the sysexit instruction requires
+ * for %eip. Thus, exactly as for arg2, we must push and pop.
+ *
+ * Arg6 is different. The caller puts arg6 in %ebp. Since the sysenter
+ * instruction clobbers %esp, the user's %esp won't even survive entry
+ * into the kernel. We store %esp in %ebp. Code in entry.S must fetch
+ * arg6 from the stack.
+ */
.text
.globl __kernel_vsyscall
.type __kernel_vsyscall,@function
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