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

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Date:	Wed, 2 Jun 2010 13:21:44 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Joerg Roedel <joro@...tes.org>
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 12:19:40PM +0200, Joerg Roedel wrote:
> 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.

devices might not be able to recover from this.

> > 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.

What you describe below does 3 ioctls for what can be done with 1.

> 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 */

Or maybe it destroys mapping for dev1?
How do you remember?

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

Also, no way to unshare? That seems limiting.

-- 
MST
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