linux-kernel - Re: [RFC -tip] x86,apic -- reduce disable

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Date:	Wed, 8 Jul 2009 18:44:52 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	"Maciej W. Rozycki" <macro@...ux-mips.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Yinghai Lu <yinghai@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC -tip] x86,apic -- reduce disable_apic usage

[Maciej W. Rozycki - Wed, Jul 08, 2009 at 12:49:11AM +0100]
| On Sun, 5 Jul 2009, H. Peter Anvin wrote:
| 
| > >  How do you set cpu_has_apic for systems with discrete local APICs?  The 
| > > CPUID flag is not set in this case.
| > > 
| > 
| > Well, should it be?  We do set flags when they're appropriate to us, and
| > if the semantics are such as that is inappropriate we can set a custom bit.
| 
|  Hmm, that might simplify things here and there and the less special cases 
| in code -- and thus effort needed -- for the discrete APIC, the better.  
| I think there is no reason why it couldn't be done -- all the places which 
| need version-specific APIC features have to check the LVR register anyway.  
| And the availability of the APICBASE MSR has to be validated separately 
| too as it comes with P6+ only.
| 
|  The only place which could care I believe is code to set X86_FEATURE_11AP 
| -- this should obviously be disabled for the discrete APIC as it is now, 
| as the chip does not suffer from the erratum and the workaround is costly 
| performance-wise.  That piece of code would have to be checked -- I don't 
| know what the order of setting of these bits would be and thus if one 
| could affect the other.  The dependency would better be well documented 
| then too -- my observation is the knowledge about the APIC subsystem among 
| people typically only covers a narrow subset of implementations.
| 
|   Maciej
| 

Thanks a lot for hints, Maciej! I've had an idea to set this bit
in verify_local_APIC (or something like that) since at this point
if discrete APIC happens -- we already complained in case of APIC
related BIOS problems. So that check-point should be safe. Anyway,
will recheck and put a big comment into patch.

	-- Cyrill
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