Re: [Qemu-devel] [0.14?][PATCH 3/4] ioapic: Prepare for base address relocation

[Blue Swirl <blauwirbel@gmail.com>]

On Thu, Feb 3, 2011 at 7:25 PM, Jan Kiszka <jan.kiszka@siemens.com> wrote:
> On 2011-02-03 20:11, Blue Swirl wrote:
>> On Thu, Feb 3, 2011 at 7:06 PM, Jan Kiszka <jan.kiszka@siemens.com> wrote:
>>> On 2011-02-03 20:01, Blue Swirl wrote:
>>>> On Thu, Feb 3, 2011 at 6:01 PM, Jan Kiszka <jan.kiszka@siemens.com> wrote:
>>>>> On 2011-02-03 18:54, Blue Swirl wrote:
>>>>>> On Thu, Feb 3, 2011 at 5:43 PM, Jan Kiszka <jan.kiszka@siemens.com> wrote:
>>>>>>> On 2011-02-03 18:36, Blue Swirl wrote:
>>>>>>>> On Thu, Feb 3, 2011 at 5:18 PM, Jan Kiszka <jan.kiszka@siemens.com> wrote:
>>>>>>>>> On 2011-02-03 18:03, Blue Swirl wrote:
>>>>>>>>>> On Thu, Feb 3, 2011 at 2:55 PM, Jan Kiszka <jan.kiszka@siemens.com> wrote:
>>>>>>>>>>> The registers of real IOAPICs can be relocated during runtime (via
>>>>>>>>>>> chipset registers). We don't support this yet, but qemu-kvm carries the
>>>>>>>>>>> current base address in its version 2 vmstate.
>>>>>>>>>>>
>>>>>>>>>>> To align both implementations for migratability, add the proper
>>>>>>>>>>> infrastructure to accept initial as well as updated base addresses and
>>>>>>>>>>> include the current address in the vmstate. This is done in a way that
>>>>>>>>>>> will also allow multiple IOAPICs in the future.
>>>>>>>>>>
>>>>>>>>>> Nack, the addresses should be device properties.
>>>>>>>>>
>>>>>>>>> Hmm.... we could make default_base_address a property. Will change that.
>>>>>>>>> But current_base_address is just the same as apicbase and can't be a
>>>>>>>>> property.
>>>>>>>>
>>>>>>>> Oh, right. What will current_base_address used for? Why can't board
>>>>>>>> just unmap IOAPIC from current address and remap it at the new
>>>>>>>> address? Then the device would not need to know its base address.
>>>>>>>
>>>>>>> The board could do this. The question is where we put this service, in
>>>>>>> the context if the IOAPIC as ioapic_set_base_address (compare to
>>>>>>> cpu_set_apic_base - which is buggy as it lacks sysbus_mmio_map) or into
>>>>>>> each and every board code. In the latter case, the boards would also be
>>>>>>> responsible for saving/restoring the address.
>>>>>>
>>>>>> How is the device relocated? Where are the chipset registers you mention?
>>>>>
>>>>> Intel's PIIX chipsets contain a register called APICBASE (but it means
>>>>> the IOAPIC), and that defines the location. The analogy in the APIC
>>>>> world is the MSR_IA32_APICBASE which we maintain via the APIC state.
>>>>
>>>> In ICH10 the register is called OIC—Other Interrupt Control Register
>>>> and the interesting bits APIC Range Select (ASEL).
>>>>
>>>> So actually PIIX should manage IOAPIC mapping, not board level.
>>>
>>> The point is we need ioapic_set_base_address logic in multiple places
>>> (once chipsets start to implement it). Better push it to a central place
>>> from the beginning. Also the bit keeping. There is no difference to
>>> apicbase.
>>
>> In that case, the function should be made inline version in ioapic.h.
>
> That still replicates the bit keeping.
>
> I don't see the benefit of moving it over, even less when we want to
> consolidate with a vmstate layout that is already in use.

The benefit is that the device model is improved.