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

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Message-ID: <5591A103.6020104@zytor.com>
Date:	Mon, 29 Jun 2015 12:48:19 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>
CC:	Mike Galbraith <umgwanakikbuti@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andy Lutomirski <luto@...nel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>, Dave Hansen <dave@...1.net>
Subject: Re: [all better] Re: regression: massive trouble with fpu rework

On 06/29/2015 02:35 AM, Ingo Molnar wrote:
> 
> Indeed, I bet that makes a difference!
> 
> I wish that 'unmasking' logic came with more comments:
> 
>   - Why do BIOSen ever mask CPUIDs?
> 

To work around bugs in legacy operating systems.

>   - Why do we unmask the masking?

Because we don't have those specific bugs.

>   - Why doesn't the kernel keep on working just fine even if certain CPUID aspects 
>     are turned off?

Because it exercises code paths that are otherwise impossible, for
example, it exposes the XSAVE capability without exposing the XSAVE
information in higher CPUID leaves.

The other option would be to have a list of CPU features that should be
turned off whenever the CPUID leaf maximum is too low, but it gives a
better user experience to just override the BIOS capping and then we
have fewer code paths in the kernel to worry about.

	-hpa

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