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

[Kirti Wankhede]

Alex,
Thanks for summarizing the discussion.

On 8/31/2016 9:18 PM, Alex Williamson wrote:
> 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.
> 

Above directory hierarchy looks fine to me. Along with the fixed set of
parameter, a optional field of extra parameter is also required. Such
parameters are required for some specific testing or running benchmarks,
for example to disable FRL (framerate limiter) or to disable console vnc
when not required. Libvirt don't need to know its details, its just a
string that user can provide and libvirt need to pass the string as it
is to vendor driver, vendor driver would act accordingly.

>>>>
>>>> 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).
> 

The directory structure for a physical GPU will be defined when device
is register to mdev module. It would be simpler to change creatable
instance count i.e for the types which can't be created creatable
instance count would be set to 0.


>>>> 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.
>>

Sorry if that was there in libvirt discussion, but "destroy" don't need
extra parameters. Yes it could be moved to mdev device directory.

>>>> 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.
> 

When mdev module creates mdev device, mdev_device_create() in patch,
here 'mdev->dev.parent' is assigned as its parent physical device. So
device_register() create child's directory inside parent's directory.
Directory for mdev device is not explicitly created. So I don't think we
can move this directory to type directory. But we can think of adding
link to type directory from mdev device's 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.
>>>>

Removing type directory dynamically seems difficult. So the other way as
suggested here, when that type is not supported, vendor driver can
return max_instance to 0.

>>>> 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!
> 

In this v7 version of patch, I had made changes that introduce 'online'
in mdev device directory as discussed in v6 reviews. We need this to
commit resources for that device(s).


>> 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.  

Group UUID would also work, as long as its unique and set for all
devices in a group, it should work.

> 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.
> 

I understand your concerns here. But making implicit doesn't guarantee
that device will not be accessed unless all mdev devices are started.

>>>> 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)
>>>>

Problem is resources should be committed before any device being
accessed and not at fault at mmio space.

>>>> 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,
>

Yes, I too agree. This framework has evolved so much and taking good
shape now. I hope we settle down on kernel and libvirt interface soon
and get this working soon :). Thanks for your support and guidance.

Thanks,
Kirti.


> Alex
>