linux-kernel - Re: [PATCH] x86/sfi: fix ioapic gsi range

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Message-ID: <m1wruacosr.fsf@fess.ebiederm.org>
Date:	Mon, 07 Jun 2010 18:10:12 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Jacob Pan <jacob.jun.pan@...ux.intel.com>,
	Alan Cox <alan@...ux.intel.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Feng Tang <feng.tang@...el.com>,
	Len Brown <len.brown@...el.com>
Subject: Re: [PATCH] x86/sfi: fix ioapic gsi range

"H. Peter Anvin" <hpa@...or.com> writes:

> On 06/07/2010 05:24 PM, Eric W. Biederman wrote:
>> Jacob Pan <jacob.jun.pan@...ux.intel.com> writes:
>> 
>>> SFI based platforms should have zero based gsi_base for IOAPICs found in SFI
>>> tables. The current code sets gsi_base starting from 1 when registering ioapic.
>>> The result is that Moorestown platform would have wrong mp_gsi_routing for each
>>> ioapic.
>> 
>> Yes starting at 1 is a bug.
>> 
>>> Background:
>>> In Moorestown/Medfield platforms, there is no legacy IRQs, all gsis and irqs
>>> are one to one mapped, including those < 16. Specifically, IRQ0 and IRQ1 are
>>> used for per-cpu timers. So without this patch, IOAPIC pin to IRQ mapping is
>>> off by one.
>> 
>> The patch looks mostly reasonable the comment is wrong.
>> 
>> You may not use a 1-1 mapping if you don't have legacy irqs.  Linux
>> irqs 0-15 are the ISA irqs you may not use those irq numbers for
>> something different on any architecture, but especially not on x86.
>> The gsi numbers are firmware specific and you may treat however you want.
>> 
>> Does the following patch work for you?
>> 
>> It appears I goofed when it was pointed out that gsi_end was inclusive and
>> didn't change the initialize.
>> 
>> diff --git a/arch/x86/kernel/apic/io_apic.c b/arch/x86/kernel/apic/io_apic.c
>> index 33f3563..5de84e5 100644
>> --- a/arch/x86/kernel/apic/io_apic.c
>> +++ b/arch/x86/kernel/apic/io_apic.c
>> @@ -90,7 +90,7 @@ int nr_ioapics;
>>  struct mp_ioapic_gsi  mp_gsi_routing[MAX_IO_APICS];
>>  
>>  /* The last gsi number used */
>> -u32 gsi_end;
>> +u32 gsi_end = -1; 
>>  
>
> This seems like asking for signedness problems, especially since this is
> used in range compares all the time.  The real problem here is that
> gsi_end is inclusive, which is almost always the wrong thing for the
> endpoint of a range.  Instead we should have the last number used plus
> one; perhaps it should be called gsi_next or gsi_free.

That does sound better.  Let me see if I can find a few minutes to implement
it that way.

Eric
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