linux-kernel - Re: [PATCH 0/1] uio_pci_generic: extensions to allow access for non-privileged processes

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Date:	Thu, 1 Apr 2010 22:09:10 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Tom Lyon <pugs@...n-about.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/1] uio_pci_generic: extensions to allow access for
	non-privileged processes

On Thu, Apr 01, 2010 at 12:18:27PM -0700, Tom Lyon wrote:
> On Thursday 01 April 2010 09:07:47 am Joerg Roedel wrote:
> > For the KVM use-case we need to be able to specify the io virtual
> > address for a given process virtual address. This is not possible with
> > the dma-api interface. So if we want to have uio-dma without an hardware
> > iommu we need two distinct interfaces for userspace to cover all
> > use-cases. I don't think its worth it to have two interfaces.
> 
> I started to add that capability but then realized that the IOMMU API also 
> doesn't allow it. The map function allows a range of physically contiguous 
> pages, not virtual.

The IOMMU-API allows that. You have to convert the user-virtual
addresses into physical addresses first. The current KVM code
already does this and uses the IOMMU-API later. You can have a look at
the gfn_to_pfn() function for a way to implement this.

> My preferred approach would be to add a DMA_ATTR that would request
> allocation of DMA at a specific device/iommu address.

No, that would be feature duplication between both APIs. Not to mention
the implementation hell this additional dma-api feature would cause for
the iommu driver developers.

	Joerg

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