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

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Message-ID: <d892d7bb-2a66-497c-4728-138c8e5e077f@intel.com>
Date:   Tue, 18 Dec 2018 13:44:41 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andi Kleen <ak@...ux.intel.com>,
        "Li, Aubrey" <aubrey.li@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Aubrey Li <aubrey.li@...el.com>, mingo@...hat.com,
        peterz@...radead.org, hpa@...or.com, tim.c.chen@...ux.intel.com,
        arjan@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 1/3] x86/fpu: track AVX-512 usage of tasks

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.