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

[Yoder Stuart-B08248]

> -----Original Message-----
> From: Yoder Stuart-B08248
> Sent: Thursday, September 05, 2013 12:51 PM
> To: Wood Scott-B07421; Sethi Varun-B16395; Bhushan Bharat-R65777; 'Peter
> Maydell'; 'Santosh Shukla'; 'Alex Williamson'; 'Alexander Graf';
> 'Antonios Motakis'; 'Christoffer Dall'; 'address@hidden'
> Cc: address@hidden; 'address@hidden'; 'qemu-
> address@hidden'
> Subject: vfio for platform devices - 9/5/2012 - minutes
> 
> We had a call with those interested and/or working on vfio
> for platform devices.
> 
> Participants: Scott Wood, Varun Sethi, Bharat Bhushan, Peter Maydell,
>               Santosh Shukla, Alex Williamson, Alexander Graf,
>               Antonios Motakis, Christoffer Dall, Kim Phillips,
>               Stuart Yoder
> 
> Several aspects to vfio for platform devices:
> 
> 1. IOMMU groups
> 
>  -iommu driver needs to register a bus notifier for the platform bus
>   and create groups for relevant platform devices
>  -Antonios is looking at this for several ARM IOMMUs
>  -PAMU (Freescale) driver already does this
> 
> 2. unbinding device from host
> 
>  PCI:
>    echo 0000:06:0d.0 > /sys/bus/pci/devices/0000:06:0d.0/driver/unbind
>  Platform:
>    echo ffe101300.dma >
> /sys/bus/platform/devices/ffe101300.dma/driver/unbind
> 
>  -don't believe there are issues or work to do here
> 
> 3. binding device to vfio-platform driver
> 
>  PCI:
>    echo 1102 0002 > /sys/bus/pci/drivers/vfio-pci/new_id
> 
>  -this is probably the least understood issue-- platform drivers
>   register themselves with the bus for a specific "name"
>   string.  That is matched with device tree "compatible" strings
>   later to bind a device to a driver
>  -we want is to have the vfio-platform driver to dynamically bind
>   to a variety of platform devices previously unknown to
>   vfio-platform
>  -ideally unbinding and binding could be an atomic operation
>  -Alex W pointed out that x86 could leverage this work so
>   keep that in mind in what we design
>  -Kim Phillips (Linaro) will start working on this

One thing we didn't discuss needs to be considered (probably by
Kim who is looking at the 'binding device' issue) is around
returning a passthru device back to the host.

After a platform device has been bound to vfio and is in use by
user space or a virtual machine, we also need to be able
to unwind all that and return the device back to the host
in a sane state.

What happens when user space exits and the vfio file
descriptors are closed?

What if the device is still active and doing bus 
mastering?   (e.g. a VM crashed causing a QEMU
exit)

How can the vfio-platform layer in the host kernel
get a specific device in a sane state?

When a plaform device is 'unbound' from vfio, what
specifically happens to the device?

Platform devices don't have generic mechanisms like on PCI
to disable bus mastering or even disable or reset a
device.

Haven't thought through all this yet, but just raising
some issues I see.

Regards,
Stuart