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

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Date:	Tue, 1 Jun 2010 13:46:51 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Tom Lyon <pugs@...co.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, chrisw@...s-sol.org, joro@...tes.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 Tue, Jun 01, 2010 at 01:28:48PM +0300, Avi Kivity wrote:
> On 06/01/2010 12:55 PM, Michael S. Tsirkin wrote:
>>
>>>   It can't program the iommu.
>>> What
>>> the patch proposes is that userspace tells vfio about the needed
>>> mappings, and vfio programs the iommu.
>>>      
>> There seems to be some misunderstanding.  The userspace interface
>> proposed forces a separate domain per device and forces userspace to
>> repeat iommu programming for each device.  We are better off sharing a
>> domain between devices and programming the iommu once.
>>    
>
>   iommufd = open(/dev/iommu);
>   ioctl(iommufd, IOMMUFD_ASSIGN_RANGE, ...)
>   ioctl(vfiofd, VFIO_SET_IOMMU, iommufd)
>
> ?

Yes.

> If so, I agree.

Good.

>> The natural way to do this is to have an iommu driver for programming
>> iommu.
>>
>> This likely means we will have to pass the domain to 'vfio' or uio or
>> whatever the driver that gives userspace the access to device is called,
>> but this is only for security, there's no need to support programming
>> iommu there.
>>
>> And using this design means the uio framework changes
>> required would be minor, so we won't have to duplicate code.
>>    
>
> Since vfio would be the only driver, there would be no duplication.  But  
> a separate object for the iommu mapping is a good thing.  Perhaps we can  
> even share it with vhost (without actually using the mmu, since vhost is  
> software only).

Main difference is that vhost works fine with unlocked
memory, paging it in on demand. iommu needs to unmap
memory when it is swapped out or relocated.

> -- 
> error compiling committee.c: too many arguments to function
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