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

[Tom Lyon <pugs@lyon-about.com>]

On Wednesday 02 June 2010 10:46:15 am Chris Wright wrote:
> * Joerg Roedel (joro@8bytes.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.

Actually, it a domain object - which may be usable among iommus (Joerg?).
However, you can't really do the dma mapping with just the domain because
every device supports a different size address space as a master, i.e.,
the dma_mask.

And I don't know how kvm would deal with devices with varying dma mask support,
or why they'd be in the same domain.