linux-kernel - Re: [PATCH RFC] x86/acpi: don't ignore I/O APICs just because there's no local APIC

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Message-ID: <4A3AACFD.5020805@goop.org>
Date:	Thu, 18 Jun 2009 14:09:17 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Keir Fraser <keir.fraser@...citrix.com>
Subject: Re: [PATCH RFC] x86/acpi: don't ignore I/O APICs just because there's
 no local APIC

On 06/18/09 13:28, Eric W. Biederman wrote:
>>> How does Xen handle domU with hardware directly mapped?
>>>   
>>>       
>> We call that "pci passthrough".  Dom0 will bind the gsi to a pirq as
>> usual, and then pass the pirq through to the domU.  The domU will bind
>> the pirq to an event channel, which gets mapped to a Linux irq and
>> handled as usual.
>>     
>
> Interesting.  How does domU find out the pirq -> pci device mapping?
>   

Hm, I haven't looked at it closely, but conventionally it would be via
xenbus (which is how all the split frontend-backend drivers communicate).

>> It is already; once the pirq is prepared, the process is the same in
>> both cases.
>>     
>
> I 3/4 believe that.  map_domain_pirq  appears to setup a per domain
> mapping between the hardware vector and the irq name it is known as.
> So I don't see how that works for other domains.
>
> msi is setup on a per domain basis.
>   

Ah, OK.  The pirq is set up for a specific domain rather than being
global (otherwise it would need some kind of "which domain can access
which pirq" table).  dom0 can either create a pirq for itself or someone
else, and the final user of the pirq binds it to a domain-local evtchn.

I think.  I really haven't looked into the pci-passthrough parts very
closely yet.

    J
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