Re: [PATCH 3/4] KVM: PPC: Add support for IOMMU in-kernel handling

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On 05/26/2013 09:44:24 PM, Alexey Kardashevskiy wrote:
On 05/25/2013 12:45 PM, David Gibson wrote:
> On Wed, May 22, 2013 at 04:06:57PM -0500, Scott Wood wrote:
>> On 05/20/2013 10:06:46 PM, Alexey Kardashevskiy wrote:
>>> diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
>>> index 8465c2a..da6bf61 100644
>>> --- a/arch/powerpc/kvm/powerpc.c
>>> @@ -396,6 +396,7 @@ int kvm_dev_ioctl_check_extension(long ext)
>>> +++ b/arch/powerpc/kvm/powerpc.c
>>> 		break;
>>> #endif
>>> 	case KVM_CAP_SPAPR_MULTITCE:
>>> +	case KVM_CAP_SPAPR_TCE_IOMMU:
>>> 		r = 1;
>>> 		break;
>>> 	default:
>>
>> Don't advertise SPAPR capabilities if it's not book3s -- and
>> probably there's some additional limitation that would be
>> appropriate.
>
> So, in the case of MULTITCE, that's not quite right.  PR KVM can
> emulate a PAPR system on a BookE machine, and there's no reason not to
> allow TCE acceleration as well.  We can't make it dependent on PAPR
> mode being selected, because that's enabled per-vcpu, whereas these
> capabilities are queried on the VM before the vcpus are created.
>
> CAP_SPAPR_TCE_IOMMU should be dependent on the presence of suitable
> host side hardware (i.e. a PAPR style IOMMU), though.


The capability says that the ioctl is supported. If there is no IOMMU group registered, than it will fail with a reasonable error and nobody gets hurt.
What is the problem?

You could say that about a lot of the capabilities that just advertise the existence of new ioctls. :-)

Sometimes it's nice to know in advance whether it's supported, before actually requesting that something happen.

>>> @@ -939,6 +940,9 @@ struct kvm_s390_ucas_mapping {
>>> #define KVM_GET_DEVICE_ATTR	  _IOW(KVMIO,  0xe2, struct
>>> kvm_device_attr)
>>> #define KVM_HAS_DEVICE_ATTR	  _IOW(KVMIO,  0xe3, struct
>>> kvm_device_attr)
>>>
>>> +/* ioctl for SPAPR TCE IOMMU */
>>> +#define KVM_CREATE_SPAPR_TCE_IOMMU _IOW(KVMIO,  0xe4, struct
>>> kvm_create_spapr_tce_iommu)
>>
>> Shouldn't this go under the vm ioctl section?


The KVM_CREATE_SPAPR_TCE_IOMMU ioctl (the version for emulated devices) is
in this section so I decided to keep them together. Wrong?

You decided to keep KVM_CREATE_SPAPR_TCE_IOMMU together with KVM_CREATE_SPAPR_TCE_IOMMU?

-Scott
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