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

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Message-Id: <2532E417-DDD2-4E2C-9F21-3B8D9B96370D@amacapital.net>
Date:   Sun, 26 Aug 2018 07:18:52 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        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



> On Aug 26, 2018, at 7:06 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> 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.

I think you’ll have to treat exit to user and context switch as different things. For exit to user, we want to restore the *user* state, but, for context switch, we’ll need to restore *kernel* state.

Do user first as its own patch set. It’ll be less painful that way.

And someone needs to rework PKRU for this to make sense. See previous threads.