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

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Date:	Mon, 27 May 2013 16:41:51 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Alexey Kardashevskiy <aik@...abs.ru>
CC:	David Gibson <david@...son.dropbear.id.au>,
	Scott Wood <scottwood@...escale.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, kvm-ppc@...r.kernel.org,
	Alexander Graf <agraf@...e.de>,
	Paul Mackerras <paulus@...ba.org>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 3/4] KVM: PPC: Add support for IOMMU in-kernel handling

Il 27/05/2013 16:26, Alexey Kardashevskiy ha scritto:
> On 05/27/2013 08:23 PM, Paolo Bonzini wrote:
>> Il 25/05/2013 04:45, David Gibson ha scritto:
>>>>> +	case KVM_CREATE_SPAPR_TCE_IOMMU: {
>>>>> +		struct kvm_create_spapr_tce_iommu create_tce_iommu;
>>>>> +		struct kvm *kvm = filp->private_data;
>>>>> +
>>>>> +		r = -EFAULT;
>>>>> +		if (copy_from_user(&create_tce_iommu, argp,
>>>>> +				sizeof(create_tce_iommu)))
>>>>> +			goto out;
>>>>> +		r = kvm_vm_ioctl_create_spapr_tce_iommu(kvm,
>>>>> &create_tce_iommu);
>>>>> +		goto out;
>>>>> +	}
>>
>> Would it make sense to make this the only interface for creating TCEs?
>> That is, pass both a window_size and an IOMMU group id (or e.g. -1 for
>> no hardware IOMMU usage), and have a single ioctl for both cases?
>> There's some duplicated code between kvm_vm_ioctl_create_spapr_tce and
>> kvm_vm_ioctl_create_spapr_tce_iommu.
> 
> Just few bits. Is there really much sense in making one function from those
> two? I tried, looked a bit messy.

Cannot really tell without the userspace bits.  But ioctl proliferation
is what the device and one_reg APIs were supposed to avoid...

>> KVM_CREATE_SPAPR_TCE could stay for backwards-compatibility, or you
>> could just use a new capability and drop the old ioctl.
> 
> The old capability+ioctl already exist for quite a while and few QEMU
> versions supporting it were released so we do not want just drop it. So
> then what is the benefit of having a new interface with support of both types?
> 
>>  I'm not sure
>> whether you're already considering the ABI to be stable for kvmppc.
> 
> Is any bit of KVM using it? Cannot see from Documentation/ABI.

I mean the userspace ABI (ioctls).

Paolo

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