linux-kernel - Re: [patch 17/17] x86/fpu: Move FPU initialization into arch_cpu_finalize

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Date:   Fri, 1 Sep 2023 14:09:17 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Nikolay Borisov <nik.borisov@...e.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Ahmed S. Darwish" <darwi@...utronix.de>,
        Arnd Bergmann <arnd@...db.de>,
        Russell King <linux@...linux.org.uk>,
        linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
        Huacai Chen <chenhuacai@...nel.org>,
        WANG Xuerui <kernel@...0n.name>, loongarch@...ts.linux.dev,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-m68k@...ts.linux-m68k.org,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-mips@...r.kernel.org,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        linux-sh@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        sparclinux@...r.kernel.org, Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        linux-um@...ts.infradead.org,
        Richard Henderson <richard.henderson@...aro.org>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Chris Zankel <chris@...kel.net>,
        Tom Lendacky <thomas.lendacky@....com>, semenzato@...gle.com
Subject: Re: [patch 17/17] x86/fpu: Move FPU initialization into
 arch_cpu_finalize_init()

On Fri, Sep 01, 2023 at 09:02:19PM +0300, Nikolay Borisov wrote:
> 
> 
> On 1.09.23 г. 20:30 ч., Guenter Roeck wrote:
> > Hi,
> > 
> > On Wed, Jun 14, 2023 at 01:39:46AM +0200, Thomas Gleixner wrote:
> > > Initializing the FPU during the early boot process is a pointless
> > > exercise. Early boot is convoluted and fragile enough.
> > > 
> > > Nothing requires that the FPU is set up early. It has to be initialized
> > > before fork_init() because the task_struct size depends on the FPU register
> > > buffer size.
> > > 
> > > Move the initialization to arch_cpu_finalize_init() which is the perfect
> > > place to do so.
> > > 
> > > No functional change.
> > > 
> > > This allows to remove quite some of the custom early command line parsing,
> > > but that's subject to the next installment.
> > > 
> > > Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> > 
> > The backport of this patch into chromeos-5.10 and chromeos-5.15 via stable
> > relase merges is causing various Chromebooks (not all of them) to crash
> > early during boot. Subsequent fixes have not addressed the problem for us,
> > so we already reverted the patch from chromeos-5.15 and will revert it
> > from chromeos-5.10 as well.
> > 
> > I don't know if this is a Chromebook specific problem, or if it affects
> > mainline, so this is just a heads-up in case others experience similar
> > problems.
> 
> 
> Another thing - if you choose to revert the arch_finalize patch then bear in
> mind that the GDS' 'force' option is rendered inoperable as the FPU can't be
> disabled due to the way things are sequenced.

I understand, but that is still better than not being able to boot
in the first place.

Thanks,
Guenter