c2k15: guenther@ on C-states, kbind, misc improvements

Our next c2k15 report comes from Philip Guenther (guenther@), who writes,

I went to Calgary expecting to do some low-level x86 improvements and fix some issues turned up in the C-state work I had committed back in June...and that's actually what I did! Weird...

First came some a diff to move the grabbing of the kernel lock for soft-interrupts from the ASM stubs to the C routine so that mere mortals can actually push it around further to reduce locking. Then I did some further work on a diff to add support for setting GS.base (the offset of the %gs segment) for userspace threads. The issue there is making sure this won't significantly affect the performance of threads not using the feature, as there are only a few programs out there that would make use of it. I'm mostly happy with my diff, but need to run performace tests on a few more machines before I'm satisfied.

On C-states, a couple developers had reported that my C-state code didn't like their machine's C1 state, resulting in higher latency during normal operation. While their boxes' AML was indeed bogus, it's *always* safe on x86 to use the HLT instruction for C1, so I fixed the code to add a fallback C1 state doing that, and their boxes became happier. (Since the hackathon, Theo, Mike Larkin (mlarkin@), and I fixed a couple other issues, including some suspend-restore bit handling that probably fixes some problems seen after resuming, and code to suppress old-style C2/C3 states on boxes where the LAPIC will stop, which fixes at least one AMD box.)

Then Theo and kettenis@ kicked me some and said "when were you planning on committing your kbind work?" (c.f. my talk at eurobsdcon2014). A quick refresh and some test runs and the kernel side went in. Then I went to work on refreshing the ld.so and csu bits, including more gcc constraint fixes on the ASM and extending it to work on more archs. Then miod@ started heckling^Wgiving me feedback, particularly on the archs I don't actually have, and things started to move faster. The userland parts aren't in yet, but I expect they'll start going in shortly after unlock...

Spent a *lot* of time trying to simplify and make convincing a set of changes to the kernel's UNIX-domain socket garbage collector. Did a round of close review and whiteboarding with claudio@ which located a couple bugs and am now working on test cases for those.

Random bits:

• reviewed random libressl diffs
• fixed a socketpair() bug introduced by the SOCK_CLOEXEC support, and then misled jca@ on another problem until bluhm@ saved the day
• taught ktrace and kdump to handle lseek()'s return code correctly on ILP32 archs
• stubbed in support for limiting symbols exported by libraries
• experimented with using mwait when spinning on the kernel lock (results vary wildly, perhaps related to number of CPU packages)
• a few tame() tweaks but much more discussion
• practiced making hipster coffee with afresh1@'s wonderful gear

Calgary was great (first time for me) and the hackroom setup was excellent. Many thanks to Nayden, the foundation, and all involved in the setup and logistics!

A varied collection of useful work indeed. Thanks for the work and the report, Philip!