x86/asm/entry: Add comments about various syscall instructions

x86/asm/entry: Add comments about various syscall instructions

SYSCALL/SYSRET and SYSENTER/SYSEXIT have weird semantics.
Moreover, they differ in 32- and 64-bit mode.

What is saved? What is not? Is rsp set? Are interrupts disabled?
People tend to not remember these details well enough.

This patch adds comments which explain in detail
what registers are modified by each of these instructions.

The comments are placed immediately before corresponding
entry and exit points.

Signed-off-by: Denys Vlasenko <[email protected]>
Signed-off-by: Andy Lutomirski <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Will Drewry <[email protected]>
Link: http://lkml.kernel.org/r/a94b98b63527797c871a81402ff5060b18fa880a.1424989793.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <[email protected]>