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

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Date:	Wed, 2 Jun 2010 12:19:40 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Avi Kivity <avi@...hat.com>, Tom Lyon <pugs@...co.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	chrisw@...s-sol.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 Wed, Jun 02, 2010 at 12:53:12PM +0300, Michael S. Tsirkin wrote:
> On Wed, Jun 02, 2010 at 11:42:01AM +0200, Joerg Roedel wrote:

> > IMO a seperate iommu-userspace driver is a nightmare for a userspace
> > interface. It is just too complicated to use.
> 
> One advantage would be that we can reuse the uio framework
> for the devices themselves. So an existing app can just program
> an iommu for DMA and keep using uio for interrupts and access.

The driver is called UIO and not U-INTR-MMIO ;-) So I think handling
IOMMU mappings belongs there.

> > We can solve the problem
> > of multiple devices-per-domain with an ioctl which allows binding one
> > uio-device to the address-space on another.
> 
> This would imply switching an iommu domain for a device while
> it could potentially be doing DMA. No idea whether this can be done
> in a safe manner.

It can. The worst thing that can happen is an io-page-fault.

> Forcing iommu assignment to be done as a first step seems much saner.

If we force it, there is no reason why not doing it implicitly.

We can do something like this then:

dev1 = open();
ioctl(dev1, IOMMU_MAP, ...); /* creates IOMMU domain and assigns dev1 to
			        it*/

dev2 = open();
ioctl(dev2, IOMMU_MAP, ...);

/* Now dev1 and dev2 are in seperate domains */

ioctl(dev2, IOMMU_SHARE, dev1); /* destroys all mapping for dev2 and
				   assigns it to the same domain as
				   dev1. Domain has a refcount of two
				   now */

close(dev1); /* domain refcount goes down to one */
close(dev2); /* domain refcount is zero and domain gets destroyed */


	Joerg

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