Re: [PATCH] VFIO driver: Non-privileged user level PCI drivers

[Joerg Roedel <joro@8bytes.org>]

On Wed, Jun 02, 2010 at 03:25:11PM +0300, Avi Kivity wrote:
> On 06/02/2010 03:19 PM, Joerg Roedel wrote:
>>
>>> Yes.  so you do:
>>> iommu = open
>>> ioctl(dev1, BIND, iommu)
>>> ioctl(dev2, BIND, iommu)
>>> ioctl(dev3, BIND, iommu)
>>> ioctl(dev4, BIND, iommu)
>>>
>>> No need to add a SHARE ioctl.
>>>      
>> In my proposal this looks like:
>>
>>
>> dev1 = open();
>> ioctl(dev2, SHARE, dev1);
>> ioctl(dev3, SHARE, dev1);
>> ioctl(dev4, SHARE, dev1);
>>
>> So we actually save an ioctl.
>>    
>
> The problem with this is that it is assymetric, dev1 is treated  
> differently from dev[234].  It's an unintuitive API.

Its by far more unintuitive that a process needs to explicitly bind a
device to an iommu domain before it can do anything with it. If its
required anyway the binding can happen implicitly. We could allow to do
a nop 'ioctl(dev1, SHARE, dev1)' to remove the asymmetry.

Note that this way of handling userspace iommu mappings is also a lot
simpler for most use-cases outside of KVM. If a developer wants to write
a userspace driver all it needs to do is:

dev = open();
ioctl(dev, MAP, ...);
/* use device with mappings */
close(dev);

Which is much easier than the need to create a domain explicitly.

	Joerg