Re: [Qemu-devel] [RFC PATCH] vfio: Add sysfsdev property for pci & platform

[Tian, Kevin]

> From: Alex Williamson [mailto:address@hidden
> Sent: Thursday, January 21, 2016 2:07 AM
> 
> vfio-pci currently requires a host= parameter, which comes in the
> form of a PCI address in [domain:]<bus:slot.function> notation.  We
> expect to find a matching entry in sysfs for that under
> /sys/bus/pci/devices/.  vfio-platform takes a similar approach, but
> defines the host= parameter to be a string, which can be matched
> directly under /sys/bus/platform/devices/.  On the PCI side, we have
> some interest in using vfio to expose vGPU devices.  These are not
> actual discrete PCI devices, so they don't have a compatible host PCI
> bus address or a device link where QEMU wants to look for it.  There's
> also really no requirement that vfio can only be used to expose
> physical devices, a new vfio bus and iommu driver could expose a
> completely emulated device.  To fit within the vfio framework, it
> would need a kernel struct device and associated IOMMU group, but
> those are easy constraints to manage.
> 
> To support such devices, which would include vGPUs, that honor the
> VFIO PCI programming API, but are not necessarily backed by a unique
> PCI address, add support for specifying any device in sysfs.  The
> vfio API already has support for probing the device type to ensure
> compatibility with either vfio-pci or vfio-platform.
> 
> With this, a vfio-pci device could either be specified as:
> 
> -device vfio-pci,host=02:00.0
> 
> or
> 
> -device vfio-pci,sysfsdev=/sys/devices/pci0000:00/0000:00:1c.0/0000:02:00.0
> 
> or even
> 
> -device vfio-pci,sysfsdev=/sys/bus/pci/devices/0000:02:00.0
> 
> When vGPU support comes along, this might look something more like:
> 
> -device 
> vfio-pci,sysfsdev=/sys/devices/virtual/intel-vgpu/address@hidden:00:02.0
> 
> NB - This is only a made up example path, but it should be noted that
> the device namespace is global for vfio, a virtual device cannot
> overlap with existing namespaces and should not create a name prone to
> conflict, such as a simple instance number.
> 

Thanks Alex! It's a good improvement to support coming vgpu feature.
Just curious. Does the virtual device name has to include a BDF format
or it can be random strings (e.g. just "vgpu0")? In the latter case, then
overlapping chance would be small.

Thanks
Kevin