linux-kernel - Re: [PATCH net-next v3 01/17] asm: simd context helper API

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Date:   Wed, 12 Sep 2018 06:14:33 +0000
From:   Kevin Easton <kevin@...rana.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        davem@...emloft.net, gregkh@...uxfoundation.org,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Samuel Neves <sneves@....uc.pt>, linux-arch@...r.kernel.org
Subject: Re: [PATCH net-next v3 01/17] asm: simd context helper API

On Mon, Sep 10, 2018 at 07:08:21PM -0600, Jason A. Donenfeld wrote:
> Sometimes it's useful to amortize calls to XSAVE/XRSTOR and the related
> FPU/SIMD functions over a number of calls, because FPU restoration is
> quite expensive. This adds a simple header for carrying out this pattern:
> 
>     simd_context_t simd_context = simd_get();
>     while ((item = get_item_from_queue()) != NULL) {
>         encrypt_item(item, simd_context);
>         simd_context = simd_relax(simd_context);
>     }
>     simd_put(simd_context);
> 
> The relaxation step ensures that we don't trample over preemption, and
> the get/put API should be a familiar paradigm in the kernel.

Given that it's always supposed to be used like that, mightn't it be
better if simd_relax() took a pointer to the context, so the call is
just

    simd_relax(&simd_context);

?

The inlining means that there won't actually be a pointer dereference in
the emitted code.

If simd_put() also took a pointer then it could set the context back to
HAVE_NO_SIMD as well?

    - Kevin