[libvirt] [PATCH v7 0/4] Add Mediated device support

[Alex Williamson]

On Wed, 31 Aug 2016 15:04:13 +0800
Jike Song <jike.song at intel.com> wrote:

> On 08/31/2016 02:12 PM, Tian, Kevin wrote:
> >> From: Alex Williamson [mailto:alex.williamson at redhat.com]
> >> Sent: Wednesday, August 31, 2016 12:17 AM
> >>
> >> Hi folks,
> >>
> >> At KVM Forum we had a BoF session primarily around the mediated device
> >> sysfs interface.  I'd like to share what I think we agreed on and the
> >> "problem areas" that still need some work so we can get the thoughts
> >> and ideas from those who weren't able to attend.
> >>
> >> DanPB expressed some concern about the mdev_supported_types sysfs
> >> interface, which exposes a flat csv file with fields like "type",
> >> "number of instance", "vendor string", and then a bunch of type
> >> specific fields like "framebuffer size", "resolution", "frame rate
> >> limit", etc.  This is not entirely machine parsing friendly and sort of
> >> abuses the sysfs concept of one value per file.  Example output taken
> >> from Neo's libvirt RFC:
> >>
> >> cat /sys/bus/pci/devices/0000:86:00.0/mdev_supported_types
> >> # vgpu_type_id, vgpu_type, max_instance, num_heads, frl_config, framebuffer,
> >> max_resolution
> >> 11      ,"GRID M60-0B",      16,       2,      45,     512M,    2560x1600
> >> 12      ,"GRID M60-0Q",      16,       2,      60,     512M,    2560x1600
> >> 13      ,"GRID M60-1B",       8,       2,      45,    1024M,    2560x1600
> >> 14      ,"GRID M60-1Q",       8,       2,      60,    1024M,    2560x1600
> >> 15      ,"GRID M60-2B",       4,       2,      45,    2048M,    2560x1600
> >> 16      ,"GRID M60-2Q",       4,       4,      60,    2048M,    2560x1600
> >> 17      ,"GRID M60-4Q",       2,       4,      60,    4096M,    3840x2160
> >> 18      ,"GRID M60-8Q",       1,       4,      60,    8192M,    3840x2160
> >>
> >> The create/destroy then looks like this:
> >>
> >> echo "$mdev_UUID:vendor_specific_argument_list" >
> >> 	/sys/bus/pci/devices/.../mdev_create
> >>
> >> echo "$mdev_UUID:vendor_specific_argument_list" >
> >> 	/sys/bus/pci/devices/.../mdev_destroy
> >>
> >> "vendor_specific_argument_list" is nebulous.
> >>
> >> So the idea to fix this is to explode this into a directory structure,
> >> something like:
> >>
> >> ├── mdev_destroy
> >> └── mdev_supported_types
> >>     ├── 11
> >>     │   ├── create
> >>     │   ├── description
> >>     │   └── max_instances
> >>     ├── 12
> >>     │   ├── create
> >>     │   ├── description
> >>     │   └── max_instances
> >>     └── 13
> >>         ├── create
> >>         ├── description
> >>         └── max_instances
> >>
> >> Note that I'm only exposing the minimal attributes here for simplicity,
> >> the other attributes would be included in separate files and we would
> >> require vendors to create standard attributes for common device classes.  
> > 
> > I like this idea. All standard attributes are reflected into this hierarchy.
> > In the meantime, can we still allow optional vendor string in create 
> > interface? libvirt doesn't need to know the meaning, but allows upper
> > layer to do some vendor specific tweak if necessary.
> >   
> 
> Not sure whether this can done within MDEV framework (attrs provided by
> vendor driver of course), or must be within the vendor driver.

The purpose of the sub-directories is that libvirt doesn't need to pass
arbitrary, vendor strings to the create function, the attributes of the
mdev device created are defined by the attributes in the sysfs
directory where the create is done.  The user only provides a uuid for
the device.  Arbitrary vendor parameters are a barrier, libvirt may not
need to know the meaning, but would need to know when to apply them,
which is just as bad.  Ultimately we want libvirt to be able to
interact with sysfs without having an vendor specific knowledge.

> >>
> >> For vGPUs like NVIDIA where we don't support multiple types
> >> concurrently, this directory structure would update as mdev devices are
> >> created, removing no longer available types.  I carried forward  
> > 
> > or keep the type with max_instances cleared to ZERO.
> >  
> 
> +1 :)

Possible yes, but why would the vendor driver report types that the
user cannot create?  It just seems like superfluous information (well,
except for the use I discover below).

> >> max_instances here, but perhaps we really want to copy SR-IOV and
> >> report a max and current allocation.  Creation and deletion is  
> > 
> > right, cur/max_instances look reasonable.
> >   
> >> simplified as we can simply "echo $UUID > create" per type.  I don't
> >> understand why destroy had a parameter list, so here I imagine we can
> >> simply do the same... in fact, I'd actually rather see a "remove" sysfs
> >> entry under each mdev device, so we remove it at the device rather than
> >> in some central location (any objections?).  
> > 
> > OK to me.   
> 
> IIUC, "destroy" has a parameter list is only because the previous
> $VM_UUID + instnace implementation. It should be safe to move the "destroy"
> file under mdev now.
> 
> >> We discussed how this might look with Intel devices which do allow
> >> mixed vGPU types concurrently.  We believe, but need confirmation, that
> >> the vendor driver could still make a finite set of supported types,
> >> perhaps with additional module options to the vendor driver to enable
> >> more "exotic" types.  So for instance if IGD vGPUs are based on
> >> power-of-2 portions of the framebuffer size, then the vendor driver
> >> could list types with 32MB, 64MB, 128MB, etc in useful and popular
> >> sizes.  As vGPUs are allocated, the larger sizes may become unavailable.  
> >
> > Yes, Intel can do such type of definition. One thing I'm not sure is 
> > about impact cross listed types, i.e. when creating a new instance
> > under a given type, max_instances under other types would be 
> > dynamically decremented based on available resource. Would it be
> > a problem for libvirt or upper level stack, since a natural interpretation
> > of max_instances should be a static number?
> >
> > An alternative is to make max_instances configurable, so libvirt has
> > chance to define a pool of available instances with different types
> > before creating any instance. For example, initially IGD driver may 
> > report max_instances only for a minimal sharing granularity:
> > 	128MB:
> > 		max_instances (8)
> > 	256MB:
> > 		max_instances (0)
> > 	512MB:
> > 		max_instances (0)
> > 
> > Then libvirt can configure more types as:
> > 	128MB:
> > 		max_instances (2)
> > 	256MB:
> > 		max_instances (1)
> > 	512MB:
> > 		max_instances (1)
> > 
> > Starting from this point, max_instances would be static and then
> > mdev instance can be created under each type. But I'm not
> > sure whether such additional configuration role is reasonable to libvirt...  

My expectation of your example, where I'm assuming you have 1G of total
memory that can be divided between the mdev devices would be:

 128M: 8
 256M: 4
 512M: 2

If a 512M mdev device is created, this becomes:

 128M: 4
 256M: 2
 512M: 1

Creating a 128M mdev device from that becomes:

 128M: 3
 256M: 1
 512M: 0

It's not great, but I don't know how to do it better without the user
having a clear understanding of the algorithm and resources required
for each mdev device.  For instance, the size here, presumably the
framebuffer size, is just one attribute in the device directory, the
user won't know that this attribute is the key to the available
instances.

I don't particularly like the idea of a writeable max_instances, the
user can simply create instances of the type and see the results.

Just thought of another thing; do we need some way to determine the
type of an mdev device from sysfs or is this implicit knowledge for the
user that created the device?  For instance, we create a 512M device
and it becomes a child device to the parent, so we can associate to the
parent, but if we come back later, how do we know it's a 512M device?
Perhaps this is a reason to keep the type directories around and we can
cross link the device to the type and create a devices subdirectory
under each type.  Perhaps then "max_instances" becomes
"available_instances" (ie. how many left we can create) and we don't
need a "current_instances" because we can simply look in the devices
directory.

> >>
> >> We still don't have any way for the admin to learn in advance how the
> >> available supported types will change once mdev devices start to be
> >> created.  I'm not sure how we can create a specification for this, so
> >> probing by creating devices may be the most flexible model.
> >>
> >> The other issue is the start/stop requirement, which was revealed to
> >> setup peer-to-peer resources between vGPUs which is a limited hardware
> >> resource.  We'd really like to have these happen automatically on the
> >> first open of a vfio mdev device file and final release.  So we
> >> brainstormed how the open/release callbacks could know the other mdev
> >> devices for a given user.  This is where the instance number came into
> >> play previously.  This is an area that needs work.  
> > 
> > IGD doesn't have such peer-to-peer resource setup requirement. So
> > it's sufficient to create/destroy a mdev instance in a single action on
> > IGD. However I'd expect we still keep the "start/stop" interface (
> > maybe not exposed as sysfs node, instead being a VFIO API), as 
> > required to support future live migration usage. We've made prototype
> > working for KVMGT today.  

Great!

> It's good for the framework to define start/stop interfaces, but as Alex
> said below, it should be MDEV oriented, not VM oriented.
> 
> I don't know a lot about the peer-to-peer resource, but to me, although
> VM_UUID + instance is not applicable, userspace can always achieve the
> same purpose by, let us assume a mdev hierarchy, providing the VM UUID
> under every mdev:
> 
> 	/sys/bus/pci/devices/<sbdf>/mdev/
> 	|-- mdev01/
> 	|   `-- vm_uuid
> 	`-- mdev02/
> 	    `-- vm_uuid
> 
> Did I miss something?

Sure, this is just another way of doing UUID+instance.  Nit, it might
look more like:

 	/sys/bus/pci/devices/<sbdf>/mdev/
 	|-- uuid1/
 	|   `-- group_uuid
 	`-- uuid2/
 	    `-- group_uuid

Where each mdev device is actually referenced by its UUID name then
we'd have some writable attribute under the device where mdev devices
sharing the same group UUID are handled together.  There's a problem
here though that vfio doesn't know about this level of grouping, so
uuid1 and uuid2 could actually be given to different users despite the
grouping here, which results in one or both devices not working or
creating security issues.  That sort of implies that this would
necessarily need to be exposed as iommu grouping.  This factors into why
it seems like a good idea to make the start/stop implicit within the
interface.  In that way each mdev device is fungible as far as a user
like libvirt is concerned, internal details like peer-to-peer resources
are handled automatically as the devices are accessed.

> >> There was a thought that perhaps on open() the vendor driver could look
> >> at the user pid and use that to associate with other devices, but the
> >> problem here is that we open and begin access to each device, so
> >> devices do this discovery serially rather than in parallel as desired.
> >> (we might not fault in mmio space yet though, so I wonder if open()
> >> could set the association of mdev to pid, then the first mmio fault
> >> would trigger the resource allocation?  Then all the "magic" would live
> >> in the vendor driver.  open() could fail if the pid already has running
> >> mdev devices and the vendor driver chooses not to support hotplug)
> >>
> >> One comment was that for a GPU that only supports homogeneous vGPUs,
> >> libvirt may choose to create all the vGPUs in advance and handle them
> >> as we do SR-IOV VFs.  The UUID+instance model would preclude such a use
> >> case.
> >>
> >> We also considered whether iommu groups could be (ab)used for this use
> >> case, peer-to-peer would in fact be an iommu grouping constraint
> >> afterall.  This would have the same UUID+instance constraint as above
> >> though and would require some sort of sysfs interface for the user to
> >> be able to create multiple mdevs within a group.
> >>
> >> Everyone was given homework to think about this on their flights home,
> >> so I expect plenty of ideas by now ;)
> >>
> >> Overall I think mediated devices were well received by the community,
> >> so let's keep up the development and discussion to bring it to
> >> fruition.  Thanks,  
> > 
> > Thanks a lot Alex for your help on driving this discussion. Mediated device
> > technique has the potential to be used for other type of I/O virtualizations
> > in the future, not limited to GPU virtualization. So getting the core framework
> > ready earlier would be highly welcomed. :-)

I agree, there's lots of potential and it's extra incentive to create
an interface that's going to make sense long term.  Ideally we only
need to create the kernel and libvirt infrastructure once and we
can handle any type of mediated driver.  Thanks,

Alex