Re: [Qemu-devel] [for-2.11 PATCH 24/26] spapr: allow guest to update the XICS phandle

[Thomas Huth <thuth@redhat.com>]

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On 28.07.2017 06:02, David Gibson wrote:
> On Tue, Jul 25, 2017 at 08:03:06PM +0200, Greg Kurz wrote:
>> The "phandle" property of the XICS node is referenced by the "interrupt-map"
>> property of each PHB node. This is used by the guest OS to setup IRQs for
>> all PCI devices.
>>
>> QEMU uses an arbitrary value (0x1111) for this phandle, but SLOF converts
>> this value to a SLOF specific one, which is then presented to the guest OS.
>>
>> This patches introduces the new KVMPPC_H_UPDATE_PHANDLE hcall, which is used
>> by SLOF to communicate the patched phandle value back to QEMU. This value
>> is then cached and preserved accross migration until machine reset.
>>
>> This is required to be able to support PHB hotplug.
>>
>> Note, that SLOF already has some code to call KVMPPC_H_RTAS_UPDATE, so we
>> have to introduce its number even if QEMU currently doesn't implement it.
>>
>> Suggested-by: Thomas Huth <thuth@redhat.com>
>> Signed-off-by: Greg Kurz <groug@kaod.org>
> 
> Ugh.  I really, really hope we can avoid this, though I don't
> immediately see how.  Having to have two way communication between
> qemu and SLOF about the device tree contents just seems like opening
> the door to endless complexities.
> 
> This is basically a consequence of the fact that both qemu and partly
> responsible for constructing the device tree for the guest, and that's
> not easy to avoid.
> 
> Hrm.. Thomas, I know it's not really the OF way, but would it be
> feasible to change SLOF to use the phandles as supplied by qemu rather
> than creating its own?

I don't see a way to do this in an easy, clean, reasonable way. SLOF
uses pointers to internal structures as phandles all over the place. You
likely can't replace that so easily without rewriting half of the whole
device tree related code in SLOF, I guess...

 Thomas


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