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

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Message-ID: <20100602125049.GB11162@8bytes.org>
Date:	Wed, 2 Jun 2010 14:50:50 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>, Tom Lyon <pugs@...co.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	chrisw@...s-sol.org, hjk@...utronix.de, gregkh@...e.de,
	aafabbri@...co.com, scofeldm@...co.com
Subject: Re: [PATCH] VFIO driver: Non-privileged user level PCI drivers

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

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