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Date:	Fri, 08 Jan 2010 18:19:21 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
CC:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	"ebiederm@...ssion.com" <ebiederm@...ssion.com>,
	Yinghai Lu <yinghai@...nel.org>,
	"Maciej W. Rozycki" <macro@...ux-mips.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [patch] x86, apic: use 0x20 for the IRQ_MOVE_CLEANUP_VECTOR instead
 of 0x1f

On 01/08/2010 06:09 PM, Suresh Siddha wrote:
> 
> So change the IRQ_MOVE_CLEANUP_VECTOR to 0x20 and  allow 0x21-0x2f to be used
> for device interrupts. 0x30-0x3f will be used for ISA interrupts (these
> also can be migrated in the context of IOAPIC and hence need to be at a higher
> priority level than IRQ_MOVE_CLEANUP_VECTOR).
> 

You're referring to when they're accessed as IOAPIC interrupts as
opposed to ExtInt interrupts?

>  
> -/*
> - * First APIC vector available to drivers: (vectors 0x30-0xee).  We
> - * start allocating at 0x31 to spread out vectors evenly between
> - * priority levels. (0x80 is the syscall vector)
> - */
> -#define FIRST_DEVICE_VECTOR		(IRQ15_VECTOR + 1)
> -#define VECTOR_OFFSET_START		1
> -
>  #define NR_VECTORS			 256
>  
>  #define FPU_IRQ				  13
> diff --git a/arch/x86/kernel/apic/io_apic.c b/arch/x86/kernel/apic/io_apic.c
> index d5bfa29..5c090a1 100644
> --- a/arch/x86/kernel/apic/io_apic.c
> +++ b/arch/x86/kernel/apic/io_apic.c
> @@ -1162,8 +1162,8 @@ __assign_irq_vector(int irq, struct irq_cfg *cfg, const struct cpumask *mask)
>  	 * Also, we've got to be careful not to trash gate
>  	 * 0x80, because int 0x80 is hm, kind of importantish. ;)
>  	 */
> -	static int current_vector = FIRST_DEVICE_VECTOR + VECTOR_OFFSET_START;
> -	static int current_offset = VECTOR_OFFSET_START % 8;
> +	static int current_vector = FIRST_DEVICE_VECTOR;
> +	static int current_offset = 0;
>  	unsigned int old_vector;
>  	int cpu, err;
>  	cpumask_var_t tmp_mask;
> 

I'm not entirely sure I like losing this bit, even though it isn't
really necessary with your changes (VECTOR_OFFSET_START would be 0).
I'm afraid we might end up with the same buglet being "reinvented" later.

However, my most serious concern with this patch is that there is a
fairly significant change due to this patch, which is that the legacy
IRQ vectors now fall *inside* the FIRST_DEVICE_VECTOR range.  This isn't
a bad thing -- in fact, it is fundamentally the right thing to do
especially once we consider platforms which *don't* have the legacy IRQs
-- but it makes me scared of unexpected behavior changes as a result.
If you feel confident that that is not the case, could you outline why
it shouldn't be a problem?

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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