linux-kernel - Re: [Xen-devel] Re: [PATCH 1/5] xen: events: use irq_alloc_desc(_at) instead of open-coding an IRQ allocator.

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Date:	Thu, 28 Oct 2010 09:22:55 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Ian Campbell <Ian.Campbell@...citrix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>, "mingo@...e.hu" <mingo@...e.hu>,
	"tglx@...utronix.de" <tglx@...utronix.de>
Subject: Re: [Xen-devel] Re: [PATCH 1/5] xen: events: use irq_alloc_desc(_at)
 instead of open-coding an IRQ allocator.

 On 10/28/2010 05:43 AM, Stefano Stabellini wrote:
> On Tue, 26 Oct 2010, Konrad Rzeszutek Wilk wrote:
>>> In that case we should use dynamic allocation for everything.  Or try to
>>> work out distinct irq ranges for different interrupts if you really want
>>> to keep irq==gsi.
>> Some little alarm bells are ringing in the back of my head about irq != gsi.
>>
>> I think the issue was the permission. When a PCI device is allocated to the
>> PV guest, we do a bunch of xc_* calls to allow the domain to use the BARs
>> and the IRQ. I believe when the guest boots and tries to map the
>> event channel with the physical IRQ, one of the arguments is that GSI. And
>> if we provide a bogus GSI, well, we won't get the INTx to the guest.
>>
>> As you mentioned, Stefano's patch add a new element to the tuple that can
>> contain the GSI value. At which point we can make the guest IRQ != GSI,
>> as long as we can contain the <gsi, event channel> mapping present so
>> that for the hypercalls we can give it the right GSI.
>>
>> The MSI/MSI-X use a completly different mechanism that does not all
>> of this complication, so we are OK with that.
>>
>> .. snip ..
>>
>>> d) dynamically allocate all irqs for all event channel types.
>> <nods> Ok, you sold me on this idea.
>>
>
> Even though dynamic allocation might seem possible for both pirqs and
> irqs, there are some severe limitations:
>
>
> - Xen won't allocate pirq numbers lower than 16 (probably because it
> expects pirq == gsi for the first 16 gsi), so it might run out
> of pirqs if we ask Xen to always choose the pirq number for us.  As a
> consequence it is safer to keep using pirq == gsi, at least for the
> first 16 gsis. This limitation should probably be fixed in Xen, but we
> need to support older hypervisors so we cannot rely on the fix to be
> present.
>
>
> - Linux expects irq == gsi, see arch/x86/kernel/acpi/boot.c:gsi_to_irq
>
> 	/* Provide an identity mapping of gsi == irq
> 	 * except on truly weird platforms that have
> 	 * non isa irqs in the first 16 gsis.
> 	 */

Yes, we always have to identity map legacy ISA interrupts, and we should
never attempt to dynamically allocate in that region.

    J

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