linux-kernel - Re: Subject: [PATCH 1/2] x86: get back 15 vectors

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Message-ID: <4B427A8C.1040902@kernel.org>
Date:	Mon, 04 Jan 2010 15:32:28 -0800
From:	Yinghai Lu <yinghai@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Jesse Brandeburg <jesse.brandeburg@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	NetDEV list <netdev@...r.kernel.org>,
	Jesse Brandeburg <jesse.brandeburg@...el.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>
Subject: Re: Subject: [PATCH 1/2] x86: get back 15 vectors

On 01/04/2010 03:03 PM, H. Peter Anvin wrote:
> On 01/04/2010 02:01 PM, Yinghai Lu wrote:
>>>
>>>>  /*
>>>> + * First APIC vector available to drivers: (vectors 0x30-0xee) we
>>>> + * start at 0x31 to spread out vectors evenly between priority
>>>> + * levels. (0x80 is the syscall vector)
>>>> + */
>>>> +#define FIRST_DEVICE_VECTOR		(IRQ15_VECTOR + 2)
>>>> +
>>>
>>> We really should fix that so we can do +1 here instead of +2; that
>>> presumably means fixing the logic so we do something smarter than just
>>> jump over 0x80.
>>
>> we already use used_vectors to skip 0x80. so we could change that to +1?
>>
> 
> Yes, but the problem is that we *skip* 0x80, which leads to suboptimal
> allocation on systems with only a handful of vectors.
> 
> The easy solution to accomplishing what we want without wasting vector
> 0x30 is obviously to start allocation at 0x31, but not by artificially
> limiting the vector space; see the attached patch.
> 
> For what it's worth, this code(__assign_irq_vector() in
> arch/x86/kernel/apic/io_apic.c) has me somewhat confused about the use
> of the constant 8:
> 
> 		vector += 8;
> 
> The only justification that I can immediately think of is to try to
> assign exactly two sources to each priority level (since early APICs
> started losing interrupts with more than two sources per priority level.)
> 
> This is ancient code -- predates not just the git but the bk history --
> and as such I would assume that that is the motivation.

yes the patch get back 0x30, 0x38, 0x40, 0x48 etc back.

YH
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