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 18:57:24 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Tom Lyon <pugs@...n-about.com>, 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 05:25:04PM +0300, Michael S. Tsirkin wrote:
> On Wed, Mar 31, 2010 at 05:08:38PM -0700, Tom Lyon wrote:
> > uio_pci_generic has previously been discussed on the KVM list, but this patch 
> > has nothing to do with KVM, so it is also going to LKML.
> > 
> > The point of this patch is to beef up the uio_pci_generic driver so that a 
> > non-privileged user process can run a user level driver for most PCIe 
> > devices. This can only be safe if there is an IOMMU in the system with 
> > per-device domains.
> 
> Why? Per-guest domain should be safe enough.

Hardware IOMMUs don't have something like a per-guest domain ;-)
Anyway, if we want to emulate an IOMMU in the guest and make this
working for pass-through devices too we need more than one domain per
guest. Essentially we may need one domain per device.

> >  Privileged users (CAP_SYS_RAWIO) are allowed if there is 
> > no IOMMU.
> 
> qemu does not support it, I doubt this last option is worth having.

Agreed.

> For this reason, I think we should address the problem somwwhat
> differently:
> - Create a character device to represent the iommu
> - This device will handle memory locking etc
> - Allow binding this device to iommu
> - Allow other operations only after iommu is bound

Yes, something like this is needed. But I think we can implement this in
the generic uio-pci-driver. A seperate interface which basically passes
the iommu-api functions to userspace doesn't make sense because it would
also be device-centric like the uio-pci-driver.

	Joerg

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