Re: [PATCH v6 1/3] x86/fpu: track AVX-512 usage of tasks

From: Andi Kleen
Date: Tue Dec 18 2018 - 17:05:40 EST

On Tue, Dec 18, 2018 at 01:44:41PM -0800, Dave Hansen wrote:
> On 12/18/18 1:38 PM, Andi Kleen wrote:
> >> I misunderstood, you mean 32bit kernel, not 32bit machine. Theoretically 32bit
> >> kernel can use AVX512, but not sure if anyone use it like this. get_jiffies_64()
> >> includes jiffies_lock ops so not good in context switch. So I want to use raw
> >> jiffies_64 here. jiffies is a good candidate but it has wraparound overflow issue.
> >> Other time source are expensive here.
> >>
> >> Should I limit the code only running on 64bit kernel?
> > Yes making it 64bit only should be fine.
> I think I'd rather just disable AVX512 itself on 32-bit and be done with
> it. I think more than half of the ~2k of XSAVE space that it consumes
> in *every* *task* is just pure waste because it has to be 0's.
> This ~2k of extra space is also lowmem, which makes it even more valuable.

That will actually break programs.

If someone compiled binaries with -march=native on a system with AVX512
they wouldn't work anymore.

Don't think we can do it.