Re: [9front] VMX improvements + AVX

[ori@eigenstate.org]

Quoth Sigrid Solveig Haflnudttir <ftrvxmtrx@gmail.com>:
> Hello.  I made some changes to /sys/src/9/^(pc pc64) and
> /sys/src/cmd/vmx to get better support for Linux and OpenBSD emulation
> and before I push it (or not) I'd like to get more eyes (and hands) on
> it.  386 kernel should not be affected by the change at all, this is
> about amd64 specifically.
> 
> Changes were tested on two machines I have, with AVX enabled/disabled.
> No problems were found with extensive use, nor any performance issues
> detected.

This fixes the 'xgetbv' trap that I was getting with OpenBSD. Thanks!
That's been something that I've wanted to get to, and I'm thrilled
that I don't need to.

> On the host Go is using AVX successfully with this change.  I'm
> writing optimized routines related to video playback so that's yet
> another reason why this work has been done in the first place.

It's also probably worth noting that the compilers for 9legacy
have some support for more recent XMM/YMM registers:

	https://github.com/0intro/plan9-contrib/commit/94fe116949ba36db8216abed83dbad8fb84ecdf7.patch

We've diverged a bit so the merge would be a pain in the ass,
and we'd need to get the disassembly done -- but at least some
of the code has been written.
 
> * AVX/AVX2 support on amd64 for both 9front kernel itself + VMX
>   guests.  Enabled by setting "*avx=" in plan9.ini.

As mentioned in gridchat: I think we can remove this knob, or
at least turn it on by default and have '*noavx=' for debugging

> * Make vmx(1) report to guest it's running under a hypervisor.

I'm skimming the code, but don't see anything obvious. How are
you reporting to the guest?

> * Provide "fast strings" (through msr) properly to guests.
> * Rework cpuid in vmx(1).
> * A bit better timing by using tsc offset feature.  Clock is still
>   wrong but at least not THAT much.  Proper kvm clock implementation
>   in the future will address that.

I'll pull up the spec soon enough and try to make a bit more sense
of this code, but it looks awesome so far. Thanks!