Re: [Qemu-devel] vfio for platform devices - 9/5/2012 - minutes

[Scott Wood]

On Thu, 2013-09-12 at 16:23 -0500, Alexander Graf wrote:
> On 12.09.2013, at 13:10, Scott Wood wrote:
> 
> > On Thu, 2013-09-12 at 04:18 -0500, Bhushan Bharat-R65777 wrote:
> >> and device disabling is not a standard like PCI. Do you think that we 
> >> might need to do some
> >> device specific handling.
> > 
> > It would be better if we could avoid the need for device specific
> > handling.  DMA can be disabled via the IOMMU.
> > 
> > The tricky part is how to enable the device in the next user; as I wrote
> > elsewhere in the thread I think this needs to be something the user is
> > aware of, so that the user quiesces the device prior to enabling IOMMU
> > mappings.
> 
> Or resets it. I think we can make it mandatory to VFIO users (and Linux
> drivers) to reset the device prior to mapping it.

Reset is one way to quiesce it, but as Bharat pointed out not all
devices make that possible (even with device specific knowledge).  In
particular, Freescale datapath stuff has problems with this.

> QEMU needs to have device specific code to assemble its device tree fragment 
> anyways, so
> it can just as well reset the device before it enables it to DMA.

Now you're adding to the depth of knowledge QEMU needs to have about a
device.  It's not that big of a deal if it's just "set this bit and wait
for it to clear", but it could be a pain with a device that can't be
reset and has a complicated sequence needed to quiesce -- and as you
noted above, Linux drivers would need to take care of this anyway.
Doing it in QEMU would still help with non-Linux guests and avoid the
need for a hypercall to indicate when the IOMMU can be opened, though.

-Scott