linux-kernel - Re: [PATCH RFC 2/5] x86,fpu: delay FPU register loading until switch to userspace

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Date:   Mon, 03 Oct 2016 22:47:53 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH RFC 2/5] x86,fpu: delay FPU register loading until
 switch to userspace

On Mon, 2016-10-03 at 19:09 -0700, Andy Lutomirski wrote:

> > Having two separate status booleans for "registers valid"
> > and "memory valid" may make more sense.
> 
> I have no problem with the concept of "owner_ctx", and I think it's a
> perfectly reasonable data structure.  My problem with it is that it's
> subtle and knowledge of it is spread all over the place.  Just going
> with "registers valid" in a variable won't work, I think, because
> there's nowhere to put it.  We need to be able to delete a struct fpu
> while that struct fpu might have a valid copy in a different cpu's
> registers.
> 
> Anyway, feel free to tell me that I'm making this too difficult :)

How about we rename fpu_want_lazy_restore to
fpu_registers_valid()?  Problem solved :)

Then we can rename __cpu_disable_lazy_restore
to fpu_invalidate_registers(), and call that
before we modify any in-memory FPU state.

> > We can get rid of fpu.counter, since nobody uses it
> > any more.
> 
> We should definitely do this.
> 
> Maybe getting in some cleanups first (my lazy fpu deletion,
> fpu.counter removal, etc) first is the way to go.

Sounds good.  I will keep my patch 1/4 as part of the
cleanup series, and will not move on to the harder
stuff until after the cleanups.

Any other stuff I should clean up while we're there?

> > > > > 
> > You are right, read_pkru() and write_pkru() can only deal with
> > the pkru state being present in registers. Is this because of an
> > assumption in the code, or because of a hardware requirement?

read_pkru and write_pkru would be candidates for using
fpu_registers_valid, and potentially a fpu_make_registers_valid,
which restores the contents of the fpu registers from memory,
if fpu_registers_valid is not true.

Likewise, we can have an fpu_make_memory_valid to ensure the
in kernel memory copy of the FPU registers is valid, potentially
a _read and _write version that do exactly what the pstate code
wants today.

Would that make sense as an API?

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