linux-kernel - Re: Lazy FPU restoration / moving kernel_fpu

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Date:   Wed, 11 Jul 2018 16:10:33 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Andrew Lutomirski <luto@...nel.org>, dave.hansen@...ux.intel.com,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>
Subject: Re: Lazy FPU restoration / moving kernel_fpu_end() to context switch

On Wed, 2018-07-11 at 18:28 +0200, Sebastian Andrzej Siewior wrote:
> On 2018-06-15 22:33:47 [+0200], Jason A. Donenfeld wrote:
> > On Fri, Jun 15, 2018 at 8:32 PM Andy Lutomirski <luto@...nel.org>
> > wrote:
> > > quite in the form you imagined.  The idea that we've tossed
> > > around is
> > > to restore FPU state on return to user mode.  Roughly, we'd
> > > introduce
> > > a new thread flag TIF_FPU_UNLOADED (name TBD).
> > > prepare_exit_to_usermode() would notice this flag, copy the
> > > fpstate to
> > > fpregs, and clear the flag.  (Or maybe exit_to_usermode_loop() --
> > > No
> > > one has quite thought it through, but I think it should be
> > > outside the
> > > loop.)  We'd update all the FPU accessors to understand the flag.
> > 
> > Yes! This is exactly what I was thinking. Then those calls to
> > begin()
> > and end() could be placed as close to the actual FPU usage as
> > possible.
> 
> I was thinking about this myself. Did anyone try to hack something in
> the meantime? I might want to look into this, too :)

I have implemented this before, back before the
big rewrite of the syscall entry and exit code.

It seemed to work fine.

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