linux-kernel - Re: [RFC PATCH 0/3] kernel: add support for 256-bit IO access

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Message-ID: <alpine.DEB.2.21.1803201039460.6506@nanos.tec.linutronix.de>
Date:   Tue, 20 Mar 2018 10:41:08 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ingo Molnar <mingo@...nel.org>
cc:     David Laight <David.Laight@...LAB.COM>,
        'Rahul Lakkireddy' <rahul.lakkireddy@...lsio.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "ganeshgr@...lsio.com" <ganeshgr@...lsio.com>,
        "nirranjan@...lsio.com" <nirranjan@...lsio.com>,
        "indranil@...lsio.com" <indranil@...lsio.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Eric Biggers <ebiggers3@...il.com>
Subject: Re: [RFC PATCH 0/3] kernel: add support for 256-bit IO access

On Tue, 20 Mar 2018, Ingo Molnar wrote:
> * Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> > > So I do think we could do more in this area to improve driver performance, if the 
> > > code is correct and if there's actual benchmarks that are showing real benefits.
> > 
> > If it's about hotpath performance I'm all for it, but the use case here is
> > a debug facility...
> > 
> > And if we go down that road then we want a AVX based memcpy()
> > implementation which is runtime conditional on the feature bit(s) and
> > length dependent. Just slapping a readqq() at it and use it in a loop does
> > not make any sense.
> 
> Yeah, so generic memcpy() replacement is only feasible I think if the most 
> optimistic implementation is actually correct:
> 
>  - if no preempt disable()/enable() is required
> 
>  - if direct access to the AVX[2] registers does not disturb legacy FPU state in 
>    any fashion
> 
>  - if direct access to the AVX[2] registers cannot raise weird exceptions or have
>    weird behavior if the FPU control word is modified to non-standard values by 
>    untrusted user-space
> 
> If we have to touch the FPU tag or control words then it's probably only good for 
> a specialized API.

I did not mean to have a general memcpy replacement. Rather something like
magic_memcpy() which falls back to memcpy when AVX is not usable or the
length does not justify the AVX stuff at all.

Thanks,

	tglx