Re: [PATCH v3 5/5] KVM: Allow host IRQ sharing for passed-through PCI 2.3 devices

[Jan Kiszka <jan.kiszka@web.de>]

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Am 03.11.2010 10:05, Michael S. Tsirkin wrote:
> On Wed, Nov 03, 2010 at 09:51:10AM +0100, Jan Kiszka wrote:
>> Am 03.11.2010 09:43, Michael S. Tsirkin wrote:
>>> On Wed, Nov 03, 2010 at 09:11:16AM +0100, Jan Kiszka wrote:
>>>> From: Jan Kiszka <jan.kiszka@siemens.com>
>>>>
>>>> PCI 2.3 allows to generically disable IRQ sources at device level. This
>>>> enables us to share IRQs of such devices between on the host side when
>>>> passing them to a guest. This feature is optional, user space has to
>>>> request it explicitly.
>>>>
>>>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>>>
>>>
>>> I just realized something.
>>> With this patch, if guest ever looks at
>>> interrupt disable bit, it will go crazy as that bit goes on/off by
>>> itself. I guess we could have an ioctl to set/clear the bit on
>>> device, and have qemu call that on config write into command/status
>>> register.
>>
>> I understand the problem, but I don't get why the kernel should bother.
>> User space has to filter the config space access, returning precisely
>> the value of the INTx disabled bit that the guest wrote.
> 
> Yes but if guest disables INTx it should not get interrupts :)

Right, got this meanwhile. KVM-in-KVM with nested device assignment
would break otherwise - intolerable. :)

Jan


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