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

Dave Hansen
2018-12-18T21:44:46Z

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.