Re: [Qemu-devel] kvm PCI assignment & VFIO ramblings

[Joerg Roedel]

On Sat, Aug 20, 2011 at 12:51:39PM -0400, Alex Williamson wrote:
> We had an extremely productive VFIO BoF on Monday.  Here's my attempt to
> capture the plan that I think we agreed to:
> 
> We need to address both the description and enforcement of device
> groups.  Groups are formed any time the iommu does not have resolution
> between a set of devices.  On x86, this typically happens when a
> PCI-to-PCI bridge exists between the set of devices and the iommu.  For
> Power, partitionable endpoints define a group.  Grouping information
> needs to be exposed for both userspace and kernel internal usage.  This
> will be a sysfs attribute setup by the iommu drivers.  Perhaps:
> 
> # cat /sys/devices/pci0000:00/0000:00:19.0/iommu_group
> 42

Right, that is mainly for libvirt to provide that information to the
user in a meaningful way. So userspace is aware that other devices might
not work anymore when it assigns one to a guest.

> 
> (I use a PCI example here, but attribute should not be PCI specific)
> 
> From there we have a few options.  In the BoF we discussed a model where
> binding a device to vfio creates a /dev/vfio$GROUP character device
> file.  This "group" fd provides provides dma mapping ioctls as well as
> ioctls to enumerate and return a "device" fd for each attached member of
> the group (similar to KVM_CREATE_VCPU).  We enforce grouping by
> returning an error on open() of the group fd if there are members of the
> group not bound to the vfio driver.  Each device fd would then support a
> similar set of ioctls and mapping (mmio/pio/config) interface as current
> vfio, except for the obvious domain and dma ioctls superseded by the
> group fd.
> 
> Another valid model might be that /dev/vfio/$GROUP is created for all
> groups when the vfio module is loaded.  The group fd would allow open()
> and some set of iommu querying and device enumeration ioctls, but would
> error on dma mapping and retrieving device fds until all of the group
> devices are bound to the vfio driver.

I am in favour of /dev/vfio/$GROUP. If multiple devices should be
assigned to a guest, there can also be an ioctl to bind a group to an
address-space of another group (certainly needs some care to not allow
that both groups belong to different processes).

Btw, a problem we havn't talked about yet entirely is
driver-deassignment. User space can decide to de-assign the device from
vfio while a fd is open on it. With PCI there is no way to let this fail
(the .release function returns void last time i checked). Is this a
problem, and yes, how we handle that?


        Joerg

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