Re: [RFC PATCH] KVM: introduce kvm_check_device

[Andre Przywara <andre.przywara@arm.com>]

Hi Scott,

thanks for looking at the patch.

On 06/01/15 20:52, Scott Wood wrote:
> On Tue, 2015-01-06 at 16:12 +0000, Andre Przywara wrote:
>> While we can easily register and unregister KVM devices, there is
>> currently no easy way of checking whether a device has been
>> registered.
>> Introduce kvm_check_device() for that purpose and use it in two
>> existing functions. Also change the return code for an invalid
>> type number from ENOSPC to EINVAL.
>> This function will be later used by another patch set to check
>> whether a KVM_CREATE_IRQCHIP ioctl is valid.
> 
> You're checking whether a device type has been registered, not the
> device itself -- so could you make it kvm_check_device_type()?

Sure.
If you are OK with the general approach, I will include the patch in my
GICv3 KVM emulation series.

>> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
>> ---
>> Hi,
>>
>> can people comment whether there is an easier way to detect KVM
>> device registration _outside_ of virt/kvm/kvm_main.c? Using the
>> KVM_CREATE_DEVICE_TEST flag sounds like a fit, but
>> kvm_ioctl_create_device() isn't exported.
> 
> Out of curiosity, why do you need to test it from inside the kernel but
> outside kvm_main.c?

I need it from arch/arm/kvm/arm.c or alternatively virt/kvm/arm/vgic.c.
The problem is that while KVM_CREATE_DEVICE works fine with checking the
availability of the requested device, KVM_CREATE_IRQCHIP does not - and
the latter is handled in the arch specific parts of the code. At the
moment the GIC_V2 is the only IRQ chip, so it's all or nothing right
now. But very soon there will be a GIC_V3, with GIC_V2 compatibility not
always being available, so KVM_CREATE_IRQCHIP may fail although there is
an in-kernel IRQ chip available.
Instead of hacking something up I thought it would be cleaner to use the
existing framework and just map KVM_CREATE_IRQCHIP to
KVM_CREATE_DEVICE(KVM_DEV_TYPE_ARM_VGIC_V2).

Thanks,
Andre.