linux-kernel - Re: [PATCH] x86: let 32bit use apic

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Date:	Mon, 14 Jul 2008 20:48:09 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	"Maciej W. Rozycki" <macro@...ux-mips.org>
Cc:	Suresh Siddha <suresh.b.siddha@...el.com>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: let 32bit use apic_ops too

[Maciej W. Rozycki - Mon, Jul 14, 2008 at 12:46:11AM +0100]
| On Sun, 13 Jul 2008, Cyrill Gorcunov wrote:
| 
| > Guys, when I was in attempt to unify apic code first thing was -
| > renaming apic_write. Here is a patch for this - only ESR and K8
| > registers are untouched - may be usefull to apply (actually not
| > sure if it will apply without fuzz now). Wonder if this help :)
| 
|  Confirmed -- with one exception all the generic write accesses to the
| APIC absolutely have to use apic_write_around() because of the lethal
| implications of the double-write erratum of some local APIC versions
| integrated with Pentium CPUs.
| 
|  The exception is the ESR register which cannot use the function because
| of: 1. its semantics which gives side-effects on a read, 2. another
| erratum, which makes the register lose its contents on a write.  
| Therefore the approach is to avoid writes, which are architecturally
| required, altogether on Pentium CPUs, which ignore them by their
| implementation, and then use straigth apic_write() on all the newer APIC
| versions which would lose some information if a read happened before a
| write.
| 
|  The K8 does not have to use apic_write_around() for the same reasons
| x86-64 does not, as neither are hit by the double-write erratum, so all
| their processor-specific write accesses may use apic_write() to avoid a
| performance hit when used with a kernel with X86_GOOD_APIC cleared.  
| Unfortunately, the LOCK# bus access always implied by the XCHG is quite
| expensive, but still less intrusive than a sequence involving masking
| interrupts locally beforehand and then restoring the IF flag to the
| previous state afterwards.  As the APIC is local to the CPU, the grant
| should not extend outside to the external bus though.
| 
|  And last, but not least, alternatives can be used these days to patch the
| expensive XCHG instructions out with cheap MOV ones -- something that was
| not available when the workaround was designed some ten years ago.
| 
|   Maciej
| 

Maciej, but if we eliminate LOCK# by using simple MOV there will not
be guarantee for atomicity. Am I wrong?

		- Cyrill -
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