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

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Date:	Wed, 2 Jun 2010 14:19:28 +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 02:21:00PM +0300, Michael S. Tsirkin wrote:
> On Wed, Jun 02, 2010 at 01:12:25PM +0200, Joerg Roedel wrote:

> > Even if it is bound to a domain the userspace driver could program the
> > device to do dma to unmapped regions causing io-page-faults. The kernel
> > can't do anything about it.
> 
> It can always corrupt its own memory directly as well :)
> But that is not a reason not to detect errors if we can,
> and not to make APIs hard to misuse.

Changing the domain of a device while dma can happen is the same type of
bug as unmapping potential dma target addresses. We can't catch this
kind of misuse.

> > > With 10 devices you have 10 extra ioctls.
> > 
> > And this works implicitly with your proposal?
> 
> Yes.  so you do:
> iommu = open
> ioctl(dev1, BIND, iommu)
> ioctl(dev2, BIND, iommu)
> ioctl(dev3, BIND, iommu)
> ioctl(dev4, BIND, iommu)
> 
> No need to add a SHARE ioctl.

In my proposal this looks like:


dev1 = open();
ioctl(dev2, SHARE, dev1);
ioctl(dev3, SHARE, dev1);
ioctl(dev4, SHARE, dev1);

So we actually save an ioctl.

> > Remember that we still need to be able to provide seperate mappings
> > for each device to support IOMMU emulation for the guest.
> 
> Generally not true. E.g. guest can enable iommu passthrough
> or have domain per a group of devices.

What I meant was that there may me multiple io-addresses spaces
necessary for one process. I didn't want to say that every device
_needs_ to have its own address space.

> > As I wrote the domain has a reference count and is destroyed only when
> > it goes down to zero. This does not happen as long as a device is bound
> > to it.
> > 
> > 	Joerg
> 
> We were talking about UNSHARE ioctl:
> ioctl(dev1, UNSHARE, dev2)
> Does it change the domain for dev1 or dev2?
> If you make a mistake you get a hard to debug bug.

As I already wrote we would have an UNBIND ioctl which just removes a
device from its current domain. UNBIND is better than UNSHARE for
exactly the reason you pointed out above. I thought I stated that
already.

	Joerg

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