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

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Message-ID: <3983fcb4c663e8dc05f13cac32ff56ec15c3dac1.camel@surriel.com>
Date:   Sun, 26 Aug 2018 12:53:29 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Andy Lutomirski <luto@...capital.net>,
        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
Subject: Re: [PATCH v2 01/17] asm: simd context helper API

On Sun, 2018-08-26 at 07:18 -0700, Andy Lutomirski wrote:
> > 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.

For non-preemptible kernel_fpu_begin/end (which seems
like a good starting point since since it gets the
code halfway to where Thomas would like it to go), the
rules would be a little simpler:

- For exit to userspace, restore the user FPU state.
- At kernel_fpu_begin(), save the user FPU state (if still loaded).
- At context switch time, save the user FPU state (if still loaded).

> 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.

I sent Thomas the patches I worked on in the past.

That series is likely incomplete, but should be a
reasonable starting point.

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