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

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Message-ID: <20100816072805.GA22479@elte.hu>
Date:	Mon, 16 Aug 2010 09:28:05 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Yinghai Lu <yinghai@...nel.org>,
	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>
Subject: Re: [PATCH] x86: Don't write io_apic ID if it is not changed


* Eric W. Biederman <ebiederm@...ssion.com> wrote:

> Yinghai Lu <yinghai@...nel.org> writes:
> 
> > For 32bit mptable path, setup_ids_from_mpc() always write io apic id
> > register, even there is no change needed.
> >
> > So try to do that when they are different bewteen reading out and
> > mptable
> 
> What is the advantage of not writing the value?

Just guessing, but if other OSs dont do it we might surprise buggy hardware 
[and trigger erratas in them], so this would be a standard 'dont touch the hw 
unless necessary' defensive move.

Similar bugs are not unprecedented.

Thanks,

	Ingo
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