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

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Message-ID: <20180711162850.52jmzsuwegpk7rag@breakpoint.cc>
Date:   Wed, 11 Jul 2018 18:28:51 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Andrew Lutomirski <luto@...nel.org>, riel@...riel.com,
        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 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 :)

Sebastian