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

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Date:	Sun, 15 Aug 2010 12:24:46 +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 Sat, Aug 14, 2010 at 01:38:01PM -0700, Yinghai Lu wrote:
> 
> 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
> 
> Signed-off-by: Yinghai Lu <yinghai@...nel.org>
> 
> ---
>  arch/x86/kernel/apic/io_apic.c |    8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> Index: linux-2.6/arch/x86/kernel/apic/io_apic.c
> ===================================================================
> --- linux-2.6.orig/arch/x86/kernel/apic/io_apic.c
> +++ linux-2.6/arch/x86/kernel/apic/io_apic.c
> @@ -2125,7 +2125,6 @@ void __init setup_ioapic_ids_from_mpc(vo
>  			physids_or(phys_id_present_map, phys_id_present_map, tmp);
>  		}
>  
> -
>  		/*
>  		 * We need to adjust the IRQ routing table
>  		 * if the ID changed.
> @@ -2137,9 +2136,12 @@ void __init setup_ioapic_ids_from_mpc(vo
>  						= mp_ioapics[apic_id].apicid;
>  
>  		/*
> -		 * Read the right value from the MPC table and
> -		 * write it into the ID register.
> +		 * Update the ID register according to the right value from
> +		 *  the MPC table if they are different.
>  		 */
> +		if (mp_ioapics[apic_id].apicid == reg_00.bits.ID)
> +			continue;
> +
>  		apic_printk(APIC_VERBOSE, KERN_INFO
>  			"...changing IO-APIC physical APIC ID to %d ...",
>  			mp_ioapics[apic_id].apicid);

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
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