[PATCH v4 15/19] arm/arm64: KVM: add virtual GICv3 distributor emulation

[Andre Przywara]

On 02/12/14 17:06, Marc Zyngier wrote:
> On 02/12/14 16:24, Andre Przywara wrote:
>> Hej Christoffer,
>>
>> On 30/11/14 08:30, Christoffer Dall wrote:
>>> On Fri, Nov 28, 2014 at 03:24:11PM +0000, Andre Przywara wrote:
>>>> Hej Christoffer,
>>>>
>>>> On 25/11/14 10:41, Christoffer Dall wrote:
>>>>> Hi Andre,
>>>>>
>>>>> On Mon, Nov 24, 2014 at 04:00:46PM +0000, Andre Przywara wrote:
>>>>>
>>>>
>>
>> [...]
>>
>>>>>>>> +
>>>>>>>> +     if (!is_in_range(mmio->phys_addr, mmio->len, rdbase,
>>>>>>>> +         GIC_V3_REDIST_SIZE * nrcpus))
>>>>>>>> +             return false;
>>>>>>>
>>>>>>> Did you think more about the contiguous allocation issue here or can you
>>>>>>> give me a pointer to the requirement in the spec?
>>>>>>
>>>>>> 5.4.1 Re-Distributor Addressing
>>>>>>
>>>>>
>>>>> Section 5.4.1 talks about the pages within a single re-distributor having
>>>>> to be contiguous, not all the re-deistributor regions having to be
>>>>> contiguous, right?
>>>>
>>>> Ah yes, you are right. But I still think it does not matter:
>>>> 1) We are "implementing" the GICv3. So as the spec does not forbid this,
>>>> we just state that the redistributor register maps for each VCPU are
>>>> contiguous. Also we create the FDT accordingly. I will add a comment in
>>>> the documentation to state this.
>>>>
>>>> 2) The kernel's GICv3 DT bindings assume this allocation is the default.
>>>> Although Marc added bindings to work around this (stride), it seems much
>>>> more logical to me to not use it.
>>>
>>> I don't disagree (and never have) with the fact that it is up to us to
>>> decide.
>>>
>>> My original question, which we haven't talked about yet, is if it is
>>> *reasonable* to assume that all re-distributor regions will always be
>>> contiguous?
>>>
>>> How will you handle VCPU hotplug for example?
>>
>> As kvmtool does not support hotplug, I haven't thought about this yet.
>> To me it looks like userland should just use maxcpus for the allocation.
>> If I get the current QEMU code right, there is room for 127 GICv3 VCPUs
>> (2*64K per VCPU + 64K for the distributor in 16M space) at the moment.
>> Kvmtool uses a different mapping, which allows to share 1G with virtio,
>> so the limit is around 8000ish VCPUs here.
>> Are there any issues with changing the QEMU virt mapping later?
>> Migration, maybe?
>> If the UART, the RTC and the virtio regions are moved more towards the
>> beginning of the 256MB PCI mapping, then there should be space for a bit
>> less than 1024 VCPUs, if I get this right.
>>
>>> Where in the guest
>>> physical memory map of our various virt machines should these regions
>>> sit so that we can allocate anough re-distributors for VCPUs etc.?
>>
>> Various? Are there other mappings than those described in hw/arm/virt.c?
>>
>>> I just want to make sure we're not limiting ourselves by some amount of
>>> functionality or ABI (redistributor base addresses) that will be hard to
>>> expand in the future.
>>
>> If we are flexible with the mapping at VM creation time, QEMU could just
>> use a mapping depending on max_cpus:
>> < 128 VCPUs: use the current mapping
>> 128 <= x < 1020: use a more compressed mapping
>>> = 1020: map the redistributor somewhere above 4 GB
>>
>> As the device tree binding for GICv3 just supports a stride value, we
>> don't have any other real options beside this, right? So how I see this,
>> a contiguous mapping (with possible holes) is the only way.
> 
> Not really. The GICv3 binding definitely supports having several regions
> for the redistributors (see the binding documentation). This allows for
> the pathological case where you have N regions for N CPUs. Not that we
> ever want to go there, really.

Ah yes, thanks for pointing that out. I was mixing this up with the
stride parameter, which is independent of this. Sorry for that.

So from a userland point of view we probably would like to have the
first n VCPU's redistributors mapped at their current places and allow
for more VCPUs to use memory above 4 GB.
Which would require quite some changes to the code to support this in a
very flexible way. I think this could be much easier if we confine
ourselves to two regions (one contiguous lower (< 4 GB) and one
contiguous upper region (>4 GB)), so we don't need to support arbitrary
per VCPU addresses, but could just use the 1st or 2nd map depending on
the VCPU number.
Is this too hackish?
If not, I would add another vgic_addr type (like
KVM_VGIC_V3_ADDR_TYPE_REDIST_UPPER or so) to be used from userland and
use that in the handle_mmio region detection.
Let me know if that sounds reasonable.

Cheers,
Andre.