linux-kernel - Re: [GIT PULL] x86 FPU changes for 5.2

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Date:   Tue, 7 May 2019 22:12:28 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Borislav Petkov <bp@...e.de>, Rik van Riel <riel@...riel.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        x86-ml <x86@...nel.org>, lkml <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] x86 FPU changes for 5.2


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Tue, May 7, 2019 at 6:26 AM Borislav Petkov <bp@...e.de> wrote:
> >
> > This branch contains work started by Rik van Riel and brought to 
> > fruition by Sebastian Andrzej Siewior with the main goal to optimize 
> > when to load FPU registers: only when returning to userspace and not 
> > on every context switch (while the task remains in the kernel).
> 
> I love this and we should have done it long ago, but I also worry that 
> every time we've messed with the FP state, we've had interesting bugs. 
> Which is obviously why we didn't do this long ago.
> 
> Has this gone through lots of testing, particularly with things like FP 
> signal handling and old machines that don't necessarily have anything 
> but the most basic FP state (ie Pentium class etc)?
> 
> I've pulled it, but I'd still like to feel safer about it 
> after-the-fact ;)

Most of the x86/fpu commits except the final one are several weeks old, 
but I have to admit that our old-systems testing is hit and miss, and FPU 
bugs do tend to have an additional 'bug report latency' multipier of a 
factor of 3 or so ...

I've been running all this (modulo the final commit) on my primary 
desktop and other systems as well, FWIIW.

Thanks,

	Ingo