Re: [PATCH v7 19/19] KVM: ARM64: Add a new kvm ARM PMU device

[Shannon Zhao <zhaoshenglong@huawei.com>]

On 2015/12/17 17:38, Marc Zyngier wrote:
> On 17/12/15 08:41, Shannon Zhao wrote:
>> > 
>> > 
>> > On 2015/12/17 16:33, Marc Zyngier wrote:
>>> >> On Thu, 17 Dec 2015 15:22:50 +0800
>>> >> Shannon Zhao <zhaoshenglong@huawei.com> wrote:
>>> >>
>>>>> >>>>
>>>>> >>>>
>>>>> >>>> On 2015/12/17 4:33, Christoffer Dall wrote:
>>>>>>> >>>>>> On Wed, Dec 16, 2015 at 04:06:49PM +0800, Shannon Zhao wrote:
>>>>>>>>> >>>>>>>> Hi,
>>>>>>>>> >>>>>>>>
>>>>>>>>> >>>>>>>> On 2015/12/16 15:31, Shannon Zhao wrote:
>>>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>> But in this case, you're returning an error if it is *not* initialized.
>>>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>> I understand that in that case you cannot return an interrupt number (-1
>>>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>> would be weird), but returning -EBUSY feels even more weird.
>>>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>> I'd settle for -ENOXIO, or something similar. Anyone having a better idea?
>>>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> >>>>>>>>>>>>>> ENXIO or ENODEV would be my choice too, and add that to the
>>>>>>>>>>>>>>> >>>>>>>>>>>>>> Documentation clearly describing when this error code is used.
>>>>>>>>>>>>>>> >>>>>>>>>>>>>>
>>>>>>>>>>>>>>> >>>>>>>>>>>>>> By the way, why do you loop over all VCPUS to set the same value when
>>>>>>>>>>>>>>> >>>>>>>>>>>>>> you can't do anything per VCPU anyway?  It seems to me it's either a
>>>>>>>>>>>>>>> >>>>>>>>>>>>>> per-VM property (that you can store on the VM data structure) or it's a
>>>>>>>>>>>>>>> >>>>>>>>>>>>>> true per-VCPU property?
>>>>>>>>>>> >>>>>>>>>> This is a per-VCPU property. PMU interrupt could be PPI or SPI. For PPI
>>>>>>>>>>> >>>>>>>>>> the interrupt numbers are same for each vcpu, while for SPI they are
>>>>>>>>>>> >>>>>>>>>> different, so it needs to set them separately. I planned to support both
>>>>>>>>>>> >>>>>>>>>> PPI and SPI. I think I should add support for SPI at this moment and let
>>>>>>>>>>> >>>>>>>>>> users (QEMU) to set these interrupts for each one.
>>>>>>>>> >>>>>>>>
>>>>>>>>> >>>>>>>> How about below vPMU Documentation?
>>>>>>>>> >>>>>>>>
>>>>>>>>> >>>>>>>> ARM Virtual Performance Monitor Unit (vPMU)
>>>>>>>>> >>>>>>>> ===========================================
>>>>>>>>> >>>>>>>>
>>>>>>>>> >>>>>>>> Device types supported:
>>>>>>>>> >>>>>>>>   KVM_DEV_TYPE_ARM_PMU_V3         ARM Performance Monitor Unit v3
>>>>>>>>> >>>>>>>>
>>>>>>>>> >>>>>>>> Instantiate one PMU instance for per VCPU through this API.
>>>>>>>>> >>>>>>>>
>>>>>>>>> >>>>>>>> Groups:
>>>>>>>>> >>>>>>>>   KVM_DEV_ARM_PMU_GRP_IRQ
>>>>>>>>> >>>>>>>>   Attributes:
>>>>>>>>> >>>>>>>>     The attr field of kvm_device_attr encodes two values:
>>>>>>>>> >>>>>>>>     bits:     | 63 .... 32 | 31 .... 0 |
>>>>>>>>> >>>>>>>>     values:   | vcpu_index |  irq_num  |
>>>>> >>>> BTW, I change this Attribute to below format and pass vcpu_index through
>>>>> >>>> this Attribute while pass irq_num through kvm_device_attr.addr.
>>>>> >>>> Is it fine?
>>>>> >>>>
>>>>> >>>>     bits:     | 63 .... 32 | 31 ....  0 |
>>>>> >>>>     values:   |  reserved  | vcpu_index |
>>> >> Using the .addr field for something that is clearly not an address is
>>> >> rather odd. Is there any prior usage of that field for something that
>>> >> is not an address?
>> > 
>> > I see this usage in vgic_attr_regs_access(). But if you prefer previous
>> > one, I'll use that.
> Ah, you're using the .addr field to point to a userspace value, not to
> carry the value itself! That'd be fine by me (even if I still prefer the
> original one), but I'd like others to chime in (I'm quite bad at
> defining userspace stuff...).

Another reason I think is that it needs to pass the irq_num to user
space when calling get_attr. It could be passed via kvm_device_attr.addr
while can't be passed via kvm_device_attr.attr within current framework.

Thanks,
-- 
Shannon