linux-kernel - Re: [PATCH V3] drivers/uio/uio_pci_generic.c: allow access for non-privileged processes

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Message-ID: <20100501142839.GD14542@8bytes.org>
Date:	Sat, 1 May 2010 16:28:40 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Tom Lyon <pugs@...n-about.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>, hjk@...utronix.de,
	gregkh@...e.de, chrisw@...s-sol.org, avi@...hat.com,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3] drivers/uio/uio_pci_generic.c: allow access for
	non-privileged processes

On Thu, Apr 29, 2010 at 12:29:40PM -0700, Tom Lyon wrote:

> I think the plan for V4 will be to take things entirely out of the UIO
> framework, and instead have a driver which supports user mode use of
> "well-behaved" PCI devices.

I really don't think that we should create another uio-subsystem for the
kernel. The current one can be easily extended to fit your and the
virtualization needs.

> So, I will go outside UIO because:

> 1 - it doesn't allow reads and writes to sub-drivers, just irqcontrol

Can you elaborate on that? What do you mean by sub-drivers?

> 2 - it doesn't have ioctls

You already added ioctls. This is not a big deal.

> 3 - it has its own interrupt model which doesn't use eventfds

Can be extended using the ioctl.

> 4 - it's ugly doing the new stuff and maintaining backwards compat.

Not really. In the ioctls you will add to enable msi/msi-x you can pass
the eventfds to userspace. Thats not ugly at all. Its just an addition.

Here is the approach I would take:

* Add an ioctl to the uio-framework and add some basic functionality
  like querying for some device capabilities like memory regions to mmap
  or io regions.
* Add ioctls to uio-pci-generic to ask for interrupt capabilities of the
  pci device (old style irqs/msi/msi-x, how many vectors and so on) and
  ioctls to enable them and get an eventfd back for each vector.

Thats a good extension and backwards compatible as well in my eyes. What
do you think?

	Joerg

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