linux-kernel - Re: [PATCH v2 01/17] asm: simd context helper API

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Message-ID: <alpine.DEB.2.21.1808261602250.1195@nanos.tec.linutronix.de>
Date:   Sun, 26 Aug 2018 16:06:32 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Andrew Lutomirski <luto@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Samuel Neves <sneves@....uc.pt>, linux-arch@...r.kernel.org,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v2 01/17] asm: simd context helper API

Jason,

On Sun, 26 Aug 2018, Jason A. Donenfeld wrote:
> On Sun, Aug 26, 2018 at 6:10 AM Thomas Gleixner <tglx@...utronix.de> wrote:
> > I'm not too fond of this simply because it requires that relax() step in
> > all code pathes. I'd rather make that completely transparent by just
> > marking the task as FPU using and let the context switch code deal with it
> > in case that it gets preempted. I'll let one of my engineers look into
> > that next week.
> 
> Do you mean to say you intend to make kernel_fpu_end() and
> kernel_neon_end() only actually do something upon context switch, but
> not when it's actually called? So that multiple calls to
> kernel_fpu_begin() and kernel_neon_begin() can be made without
> penalty?

On context switch and exit to user. That allows to keep those code pathes
fully preemptible. Still twisting my brain around the details.

> If so, that'd be great, and I'd certainly prefer this to the
> simd_context_t passing. I consider the simd_get/put/relax API a
> stopgap measure until something like that is implemented.

I really want to avoid this stopgap^Wducttape thing.

Thanks,

	tglx