x86_64,entry: Rearrange the syscall exit optimizations

Linus, I suspect you'll either like or hate this series. Or maybe you'll think it's crazy but you'll like it anyway. I'm curious which of those is the case. :)

The syscall exit asm is a big mess. There's a really fast path, some kind of fast path code (with a hard-coded optimization for audit), and the really slow path. The result is that it's very hard to work with this code. There are some asm paths that are much slower than they should be (context tracking is a major offender), but no one really wants to add even more asm to speed them up.

This series takes a different, unorthodox approach. Rather than trying to avoid entering the very slow iret path, it adds a way back out of the iret path. The result is a dramatic speedup for context tracking, user return notification, and similar code, as the cost of a few lines of tricky asm. Nonetheless, it's barely a net addition of asm code, because we get to remove the fast path optimizations for audit and rescheduling.

Thoughts? If this works, it opens the door for a lot of further consolidation of the exit code.

Note: patch 1 in this series has been floating around on the list for quite a while. It's mandatory for this series to work, because the buglet that it fixes almost completely defeats the optimization that I'm introducing.

Note: Some optimization along these lines is probably certainly needed to get anything resembling acceptable performance out of the FPU changes that Rik is working on.

Andy Lutomirski (3):
x86_64,entry: Fix RCX for traced syscalls
x86_64,entry: Use sysret to return to userspace when possible x86_64,entry: Remove the syscall exit audit and schedule optimizations

 arch/x86/kernel/entry_64.S | 109 +++++++++++++++++++++++++--------------------
 1 file changed, 61 insertions(+), 48 deletions(-)

2.1.0