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

From: Tom Lyon
Date: Thu Apr 01 2010 - 15:21:19 EST

On Thursday 01 April 2010 09:07:47 am Joerg Roedel wrote:
> On Thu, Apr 01, 2010 at 08:40:34AM -0700, Tom Lyon wrote:
> > On Thursday 01 April 2010 05:52:18 am Joerg Roedel wrote:
> > > > 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. Privileged users (CAP_SYS_RAWIO) are
> > > > allowed if there is no IOMMU.
> > >
> > > If you rely on an IOMMU you can use the IOMMU-API instead of the
> > > DMA-API for dma mappings. This change makes this driver suitable for
> > > KVM use too. If the interface is designed clever enough we can even use
> > > it for IOMMU emulation for pass-through devices.
> >
> > The use with privileged processes and no IOMMUs is still quite useful, so
> > I'd rather stick with the DMA interface.
> 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.
> Joerg

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.

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