linux-kernel - Re: [all better] Re: regression: massive trouble with fpu rework

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Message-ID: <20150627210238.GB3054@khazad-dum.debian.net>
Date:	Sat, 27 Jun 2015 18:02:38 -0300
From:	Henrique de Moraes Holschuh <hmh@....eng.br>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [all better] Re: regression: massive trouble with fpu rework

On Sat, 27 Jun 2015, Mike Galbraith wrote:
> > > BIOS setting "Limit CPUID Maximum" upsets new fpu code mightily.
> 
> That BIOS setting is annotated with the helpful text "Disabled for
> Windows XP".  It makes box say interesting things during boot, like...
> 
>         x86/fpu: XSTATE_CPUID missing!
> 
> 
> ..or with HEAD, it triggers warning..
> 
>         if (boot_cpu_data.cpuid_level < XSTATE_CPUID) {
>                 WARN_ON_FPU(1);
>                 return;
>         }
> 
> ..and all kinds of bad juju follows.  I have no idea what the thing does
> beyond what I can interpolate from the word 'limit'.

Well, it is supposed to disable CPUID levels >= 0x04.  This thing should
*NEVER* be enabled, the last operating system that required it to be enabled
was Windows 98.

Can/do we override that crap during cpu init?  If we cannot/don't, maybe
instead of limping along with CPUID crippled, it would be better to either
output a very nasty warning, or outright stop booting [with an appropriate
error message] ?

-- 
  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh
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