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

[Andy Lutomirski <luto@amacapital.net>]

> On Aug 26, 2018, at 7:06 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> 
> Jason,
> 
>> On Sun, 26 Aug 2018, Jason A. Donenfeld wrote:
>>> On Sun, Aug 26, 2018 at 6:10 AM Thomas Gleixner <tglx@linutronix.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.