linux-kernel - Re: [PATCH RFC 1/3] fpu/x86: add make_fpregs_active(

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Message-ID: <CALCETrVG2p930Rc5gpE0ynWR310fCBoxVj7KNgtbKbD=toj77Q@mail.gmail.com>
Date:   Mon, 17 Oct 2016 16:30:47 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     Rik van Riel <riel@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Lutomirski <luto@...nel.org>,
        dave.hansen@...el.linux.com, Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH RFC 1/3] fpu/x86: add make_fpregs_active(_newstate) helper functions

On Mon, Oct 17, 2016 at 4:04 PM, Yu-cheng Yu <yu-cheng.yu@...el.com> wrote:
> On Mon, Oct 17, 2016 at 01:57:06PM -0700, Andy Lutomirski wrote:
>> Dave and/or Yu-cheng: didn't one of you have some code to allow a user
>> xstate buffer to be filled from the copy in kernel memory?  If we did
>> that, we could avoid this mess entirely.
>
> In copy_fpstate_to_sigframe() (arch/x86/kernel/fpu/signal.c), the
> assumption was we have lazy fpu:
>
>         if (fpregs_active() || we want an #NM exception)
>                 copy_fpregs_to_sigframe();
>         else
>                 copy kernel buffer to user buffer;
>
> But this is not the true anymore.  Or do you mean something else?

Rik wants to add a different form of FPU laziness, and it would be
simpler if we could just always copy from a kernel buffer.  Does code
to do that exist in the tree?

--Andy