linux-kernel - Re: [PATCH] x86: Don't write io

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Message-ID: <20100816090645.GA5805@lenovo>
Date:	Mon, 16 Aug 2010 13:06:45 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH] x86: Don't write io_apic ID if it is not changed

On Sun, Aug 15, 2010 at 12:30:57PM +0400, Cyrill Gorcunov wrote:
> On Sun, Aug 15, 2010 at 12:24:46PM +0400, Cyrill Gorcunov wrote:
> ...
> > As far as I see, this was done by a purpose in former code. Consider the
> > situation when mp_ioapics[apic_id].apicid >= get_physical_broadcast().
> > By code flow (io_apic.c:2099) this set
> > 
> > 		if (mp_ioapics[apic_id].apicid >= get_physical_broadcast()) {
> > 			printk(KERN_ERR "BIOS bug, IO-APIC#%d ID is %d in the MPC table!...\n",
> > 				apic_id, mp_ioapics[apic_id].apicid);
> > 			printk(KERN_ERR "... fixing up to %d. (tell your hw vendor)\n",
> > 				reg_00.bits.ID);
> > --->			mp_ioapics[apic_id].apicid = reg_00.bits.ID;
> > 		}
> > 
> > So with your patch we always hit "continue" without real changing of ID which
> > is not correct.
> > 
> > But perhaps I miss something?
> > 
> > 	-- Cyrill
> 
> False alarm Yinghai, of course there is no need to write same value back,
> sorry for noise.
> 
> 	-- Cyrill

Thinking a bit more I guess there might be a sence to write io-apic id
unconditionally, io-apic id is not latched on reset but set to 0 (at least
for old apics) so if we hit id match -- this will be once, but the same
time we will have additional if() for every cycle iteration, not sure
it worth it. Right?

	-- Cyrill
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