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

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Message-ID: <CALCETrVW=z2fH0JHLxvSbKHx2pbkkFZdQP2OTXpivrS=robisA@mail.gmail.com>
Date:   Mon, 3 Oct 2016 20:02:45 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Rik van Riel <riel@...hat.com>
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, Oct 3, 2016 at 7:47 PM, Rik van Riel <riel@...hat.com> wrote:
> 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.

Sounds good to me.

>
>> > 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?

Almost certainly, but nothing I'm thinking of right now :)

>
>> > > > >
>> > 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?
>
> --
> All Rights Reversed.



-- 
Andy Lutomirski
AMA Capital Management, LLC