linux-kernel - Re: [patch 2/2] x86,apic: Use logical OR in noop-write operation

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Message-ID: <4B1EB2B0.4000204@zytor.com>
Date:	Tue, 08 Dec 2009 12:10:24 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
CC:	tglx@...utronix.de, mingo@...e.hu, macro@...ux-mips.org,
	yinghai@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 2/2] x86,apic: Use logical OR in noop-write operation

On 12/08/2009 07:53 AM, Cyrill Gorcunov wrote:
> For apic noop'ified we have to use logical OR statement,
> otherwise any write on systems shipped with 82489DX
> (where apic presence bit can't be retrieved via cpuid)
> will not trigger the warning which is not desired.
> 
> In turn noop'ified read operation remains using logical
> AND statement. This will catch _only_ future code bugs
> since:
> 
> 1) The old 32bit systems without apic presence bit use
>    disable_apic only as a flag that chip was turned off
>    via boot option but not due to MP-table (BIOS) bug where
>    we may try to re-enable apic via MSR registers. The further
>    SMP setup code already prepared for a such situation
>    (ie it disables SMP support) and do not issue write
>    operations but still needs to issue apic_read. So instead
>    of deforming code with "if" we just allow reads until
>    SMP support gets disabled. But even then we still need
>    apic_read issued on a such machine at shutdown procedure.
> 
> 2) x86-64 at moment properly uses cpu_has_apic and turn
>    this feature off as only chip is disabled together
>    with disable_apic flag.
> 

Could you clarify the question I asked yesterday: how is the "or"
different from just warning unconditionally (which would at least be a
lot more clear)?  Can the situation that !cpu_has_apic && disable_apic
actually happen and we *still* end up in apic_write?

	-hpa
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