Seccomp performance vs. asm complexity

[Andy Lutomirski]

This is a question about tradeoffs.  I'm playing with a couple of
approaches for an x86 seccomp fast path.

It looks like populating struct seccomp_data in the syscall entry asm
code saves 4-5ns (83ns vs 87.7ns or so for getpid with seccomp
enabled).  Presumably this is because it avoids a branch and replaces
seven two-instruction memory copies with 6 register pushes and one
memory push.  It also keeps the code shorter, with corresponding
icache benefits.

OTOH, populating struct seccomp_data in C keeps the asm code shorter
and simpler.  In fast, it ends up being a net deletion of asm code.

Thoughts?  What's a line of assembly code worth?  Keep in mind that
someone will probably want to port this to the x86_32 and compat
entries.

To keep this in perspective, this is down from >200ns in 3.16-rc3.

My current code is here:

https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/log/?h=x86/seccomp-fastpath

I think it's in pretty good shape, but I still want to play with it a
bit before sending it out.  This version uses the C approach.

--Andy
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/