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

From: H. Peter Anvin
Date: Mon Jun 07 2010 - 20:31:16 EST


On 06/07/2010 05:24 PM, Eric W. Biederman wrote:
> Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx> 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.

-hpa

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