[PATCH 18/19] KVM: PPC: Book3S HV: add passthrough support

[Cédric Le Goater]

On 1/30/19 6:55 AM, Paul Mackerras wrote:
> On Tue, Jan 29, 2019 at 06:44:50PM +0100, Cédric Le Goater wrote:
>> On 1/29/19 5:12 AM, Paul Mackerras wrote:
>>> On Mon, Jan 28, 2019 at 07:26:00PM +0100, Cédric Le Goater wrote:
>>>>
>>>> Is clearing the PTEs and repopulating the VMA unsafe ? 
>>>
>>> Actually, now that I come to think of it, there could be any number of
>>> VMAs (well, up to almost 64k of them), since once you have a file
>>> descriptor you can call mmap on it multiple times.
>>>
>>> The more I think about it, the more I think that getting userspace to
>>> manage the mappings with mmap() and munmap() is the right idea if it
>>> can be made to work.
>>
>> We might be able to mmap() and munmap() regions of ESB pages in the RTAS 
>> call "ibm,change-msi".  I think that's the right spot for it. 
> 
> I was thinking that the ESB pages should be mmapped for device
> interrupts at VM startup or when a device is hot-plugged in, and
> munmapped when the device is hot-removed.  Maybe the mmap could be
> done in conjunction with the KVM_IRQFD call?
> 
> What is the reasoning behind doing it in the ibm,change-msi call?

Because when the device is plugged in, it has no interrupts. These are 
allocated on demand only when the driver is loaded in the guest. 

C.