linux-kernel - Re: [PATCH v2 2/2] x86: don't unmask disabled irqs when migrating them

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Message-ID: <4DC74729.9090508@goop.org>
Date:	Mon, 09 May 2011 11:45:13 +1000
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"Tian, Kevin" <kevin.tian@...el.com>
CC:	Ian Campbell <Ian.Campbell@...citrix.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>,
	"JBeulich@...ell.com" <JBeulich@...ell.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>
Subject: Re: [PATCH v2 2/2] x86: don't unmask disabled irqs when migrating
 them

On 05/09/2011 10:44 AM, Tian, Kevin wrote:
>> It doesn't really need to be an irq.  The main reason was so that it would
>> appear in /proc/interrupts so I could use the counter as a "number of times a
>> spinlock was kicked" counter.  That could be exposed in some other way if
>> being part of the interrupt infrastructure brings too much baggage with it.
>>
> Perhaps we don't need an irq binding here. Just like a local APIC interrupt
> source which only needs vector. Somehow the virq or vipi concept in Xen
> context is similar.

An event channel is logically equivalent to a vector, so that would make
sense.  We currently allocate irqs for cross-cpu call and reschedule
event channels, whereas native x86 simply uses a naked vector for
those.  But they are real interrupts, so an irq at least makes some
logical sense in those cases.

For spinlocks, the event channel is more like a usermode-level signal
which is always blocked and only ever tested with sigsuspend (or is it
sigpoll?  something like that).

    J
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