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Re: [libvirt] Introduce vGPU mdev framework to libvirt

On Tue, 14 Feb 2017 16:50:14 +0100
Martin Polednik <mpolednik redhat com> wrote:

> On 07/02/17 12:29 -0700, Alex Williamson wrote:
> >On Tue, 7 Feb 2017 17:26:51 +0100
> >Erik Skultety <eskultet redhat com> wrote:
> >  
> >> On Mon, Feb 06, 2017 at 09:33:14AM -0700, Alex Williamson wrote:  
> >> > On Mon,  6 Feb 2017 13:19:42 +0100
> >> > Erik Skultety <eskultet redhat com> wrote:
> >> >  
> >> > > Finally. It's here. This is the initial suggestion on how libvirt might
> >> > > interract with the mdev framework, currently only focussing on the non-managed
> >> > > devices, i.e. those pre-created by the user, since that will be revisited once
> >> > > we all settled on how the XML should look like, given we might not want to use
> >> > > the sysfs path directly as an attribute in the domain XML. My proposal on the
> >> > > XML is the following:
> >> > >
> >> > > <hostdev mode='subsystem' type='mdev'>
> >> > >     <source>
> >> > >         <!-- this is the host's physical device address -->
> >> > >         <address domain='0x0000' bus='0x00' slot='0x00' function='0x00'>
> >> > >         <uuid>vGPU_UUID<uuid>
> >> > >     <source>
> >> > >     <!-- target PCI address can be omitted to assign it automatically -->
> >> > > </hostdev>
> >> > >
> >> > > So the mediated device is identified by the physical parent device visible on
> >> > > the host and a UUID which allows us to construct the sysfs path by ourselves,
> >> > > which we then put on the QEMU's command line.  
> >> >
> >> > Based on your test code, I think you're creating something like this:
> >> >
> >> > -device vfio-pci,sysfsdev=/sys/class/mdev_bus/0000:00:03.0/53764d0e-85a0-42b4-af5c-2046b460b1dc
> >> >
> >> > That would explain the need for the parent device address, but that's
> >> > an entirely self inflicted requirement.  For a managed="no" scenarios,
> >> > we shouldn't need the parent, we can get to the mdev device
> >> > via /sys/bus/mdev/devices/53764d0e-85a0-42b4-af5c-2046b460b1dc.  So it  
> >>
> >> True, for managed="no" would this path be a nice optimization.
> >>  
> >> > seems that the UUID should be the only required source element for
> >> > managed="no".
> >> >
> >> > For managed="yes", it seems like the parent device is still an optional  
> >>
> >> The reason I went with the parent address element (and purposely neglecting the
> >> sample mtty driver) was that I assumed any modern mdev capable HW would be
> >> accessible through the PCI bus on the host. Also I wanted to explicitly hint
> >> libvirt as much as possible which parent device a vGPU device instance should
> >> be created on in case there are more than one of them, rather then scanning
> >> sysfs for a suitable parent which actually supports the given vGPU type.
> >>  
> >> > field.  The most important thing that libvirt needs to know when
> >> > creating a mdev device for a VM is the mdev type name.  The parent
> >> > device should be an optional field to help higher level management
> >> > tools deal with placement of the device for locality or load balancing.
> >> > Also, we can't assume that the parent device is a PCI device, the
> >> > sample mtty driver already breaks this assumption.  
> >>
> >> Since we need to assume non-PCI devices and we still need to enable management
> >> to hint libvirt about the parent to utilize load balancing and stuff, I've come
> >> up with the following adjustments/ideas on how to reflect that in the XML:
> >> - still use the address element but use it with the 'type' attribute [1] (still
> >>   breaks the sample mtty driver though) while making the element truly optional
> >>   if I'm going to be outvoted in favor of scanning the directory for a suitable
> >>   parent device on our own, rather than requiring the user to provide that
> >>
> >> - providing either an attribute or a standalone element for the parent device
> >>   name, like a string version of the PCI address or whatever form the parent
> >>   device comes in (doesn't break the mtty driver but I don't quite like this)
> >>
> >> - providing a path element/attribute to sysfs pointing to the parent device
> >>   which I'm afraid is what Daniel is not in favor of libvirt doing
> >>
> >> So, this is what I've so far come up with in terms of hinting libvirt about the
> >> parent device, do you have any input on this, maybe some more ideas on how we
> >> should identify the parent device?  
> >
> >IMO, if we cannot account for the mtty sample driver, we're doing it
> >wrong.  I suppose we can leave it unspecified how one selects a parent
> >device for the mtty driver, but it should be possible to expand the
> >syntax to include it.  So I think that means that when the parent
> >address is provided, the parent address type needs to be specified as
> >PCI.  So...
> >
> > <hostdev mode='subsystem' type='mdev'>
> >
> >This needs to encompass the device API or else the optional VM address
> >cannot be resolved.  Perhaps model='vfio-pci' here?  Seems similar to
> >how we specify the device type for PCI controllers where we have
> >multiple options:
> >
> > <hostdev mode='subsystem' type='mdev' model='vfio-pci'>
> >
> >   <source>
> >
> >For managed='no', I don't see that anything other than the mdev UUID is
> >useful.
> >
> >     <uuid>MDEV_UUID</uuid>
> >
> >If libvirt gets into the business of creating mdev devices and we call
> >that managed='yes', then the mdev type to create is required.  I don't
> >know whether there's anything similar we can steal syntax from:
> >
> >     <type>"nvidia-11"</type>
> >
> >That's pretty horrible, needs some xml guru love.
> >
> >We need to provide for specifying a parent, but we can't assume the  
> From higher level perspective, I believe it would be "good
> enough" for most of the cases to only specify the type. Libvirt will
> anyway have to be able to enumerate the devices for listAllDevices
> afaik.
> My wish would be specifying
> <hostdev mode='subsystem' type='mdev'>
>     <type>nvidia-11</type>
> </hostdev>
> unless the user has specific requests or some other decision (mmio
> numa placement) takes place.

Yes, the <type> is the minimum information necessary for libvirt to
create the mdev device itself.  A <source> section could add optional
placement information.  Note though that without an nvidia-11 type
device on the system to query, the xml doesn't tell us what sort of
device this creates in the VM.  We could assume that it's vfio-pci, but
designing in an assumption isn't a great idea.  So, as above, some
mechanism to make the xml self contained, such as specifying the model
as vfio-pci, helps avoid that assumption and allows us to know the
format for expressing the VM <address>
> We would additionally need (allocated instances/max instances of that
> type) in listAllDevices to account for the specific assignment
> possibility.

mdev devices support an available_instances per mdev type that is
dynamically updated as devices are created.  The interaction of
available_instances between different types is going to require some
heuristics to understand.  Some vendors may not support heterogeneous
types, others may pull from a common pool of resources, where each type
may consume resources from that pool at different rates.
> I'm not sure what the decision was wrt type naming, can 2 different
> cards have similarly named type with different meaning?

We don't deal in similarities, each type ID is unique and it's up to
the mdev vendor driver to make sure that an "nvidia-11" on and M60 card
is software equivalent to an "nvidia-11" on an M10 card.  If they're
not equivalent, the type ID will be different.  Something we may want
to consider eventually is whether we want/need to deal with
compatibility strings.  For instance, NVIDIA seems to be tying the type
ID strongly to specific implementations, an nvidia-11 may only be
available on an M60 card.  An M10 card may offer an nvidia-21 type with
similar capabilities.  There may be a need to express an mdev device as
compatible with various type IDs for hardware availability, at the risk
of exposing slight variations to the VM.  This could also make
placement easier for vendor drivers that only support homogeneous mdev
devices, "I prefer an mdev ID of type 'nvidia-11', but will accept one
of type 'nvidia-12,nvidia-21'".  Thanks,


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