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

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Date:	Wed, 2 Jun 2010 10:46:15 -0700
From:	Chris Wright <chrisw@...s-sol.org>
To:	Joerg Roedel <joro@...tes.org>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>, 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

* Joerg Roedel (joro@...tes.org) wrote:
> 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.

This is not any hot path, so saving an ioctl shouldn't be a consideration.
Only important consideration is a good API.  I may have lost context here,
but the SHARE API is limited to the vfio fd.  The BIND API expects a new
iommu object.  Are there other uses for this object?  Tom's current vfio
driver exposes a dma mapping interface, would the iommu object expose
one as well?  Current interface is device specific DMA interface for
host device drivers typically mapping in-flight dma buffers, and IOMMU
specific interface for assigned devices typically mapping entire virtual
address space.

thanks,
-chris
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