[libvirt] Matching the type of mediated devices in the migration

[Alex Williamson]

On Tue, 31 Jul 2018 04:05:11 +0800
Zhi Wang <zhi.a.wang at intel.com> wrote:

> On 07/30/18 23:56, Alex Williamson wrote:
> > On Sun, 29 Jul 2018 21:19:41 +0000
> > "Wang, Zhi A" <zhi.a.wang at intel.com> wrote:
> >   
> >> BACKGROUND
> >>
> >> As the live migration of mdev is going to be supported in VFIO, a scheme of deciding if a mdev could be migratable between the source machine and the destination machine is needed. Mostly, this email is going to discuss a possible solution which needs fewer modifications of libvirt/VFIO.
> >>
> >> The configuration of a mdev is located in the domain XML, which guides libvirt how to find the mdev and generating the command line for QEMU. It basically only includes the UUID of a mdev. The domain XML of the source machine and destination machine are going to be compared before the migration really happens. Each configuration item would be compared and checked by libvirt. If one item of the source machine is different from the item of destination machine, the migration fails. For mdev, there is no any check/match before the migration happens yet.
> >>
> >> The user could use the node device list of libvirt to list the host devices and see the capabilities of those devices. The current node device code of libvirt has already been able to extract the supported mdev types from a host PCI device, plus some basic information, like max supported mdev instance of a host PCI device.
> >>
> >> THE SOLUTION
> >>
> >> To strictly check the mdev type and make sure the migration happens between the compatible mediated devices, three new mandatory elements in the domain XML below the hostdev element would be introduced:
> >>
> >> vendorid: The vendor ID of the mdev, which comes from the host PCI device. A user could obtain this information from the host PCI device which supports mdev in the node device list.
> >> productid: The product ID of the mdev, which also comes from the host PCI device. A user could obtain this information from the same approach above.  
> > 
> > The parent of an mdev device is not necessarily a PCI device.  
> Good point. I didn't get that.
> >   
> >> mdevtype: The type of the mdev. As the creation of the mdev is managed by the user, the user knows the type of the mdev and would be responsible for filling out this information.
> >>
> >> These three elements are only needed when the device API of a mdev is "vfio-PCI". Take the example of mdev configuration from https://libvirt.org/formatdomain.html to illustrate the modification:
> >>
> >>    <devices>
> >>      <hostdev mode='subsystem' type='mdev' model='vfio-pci'>
> >>      <source>
> >>        <address uuid='c2177883-f1bb-47f0-914d-32a22e3a8804'/>
> >>        <vendorid>0xdead</vendorid> <!-- The VID of the host PCI device which supports this mdev -->
> >>        <productid>0xbeef</productid> <!-- The PID of the host PCI device which supports this mdev -->
> >>        <mdevtype>type</mdevtype> <!-- The vendor-specific mdev type string -->
> >>      </source>
> >>      </hostdev>
> >>
> >> With the newly introduced elements above, the flow of the creation of a domain XML with mdev will be like:
> >>
> >> 1. The user obtains the vendorid/productid from node device list
> >> 2. The user fills the vendorid/productid/mdevtype in the domain XML
> >> 3. When a migration happens, libvirt check these elements. If one item is different between two domain XML, then migration fails.  
> > 
> > I don't see how this solves anything.  The vendor and product are
> > redundant and specific to PCI hosted mdev devices.  These do nothing
> > to enhance the definition of an mdev type, where we've decided the
> > mdev type is a guest software compatible definition of a device.
> > Simply knowing the type doesn't help me know that the state data
> > between source and target is compatible.  This is the difference
> > between knowing I'm migrating from machine 'pc-440fx' to 'pc-440fx'
> > versus 'pc-i440fx-2.12' to 'pc-440fx-2.11'.  We need somehow to define
> > a version of a device, what we consider to be compatible versions for
> > migration, and hopefully some standard(ish) mechanism libvirt could
> > use to determine this.  Thanks,
> >   
> 
> I see your point. We could combine these stuff together and improve 
> "mdev" type, not by introducing new stuff to decide the compatibility. 
> Let me know if I misunderstood.
> 
> I guess you are now talking about "the thing" we should give libvirt. 
> Are you implying that the mdev type we give in libvirt should be a 
> string? If we could take the inspiration of PCI device? Like:
> 
>             class name - vendor name - product name - version
> 
> mdev type  gpu-intel-gen9-11
>             gpu-nvidia-grid-11
> 
> Then every mdev driver needs to fill these information and VFIO could 
> combine and expose them as the name of folder in mdev_supported_types. 
> Libvirt could address the mdev type by reading the mdev_type in UUID folder.

I don't think this is practical, the mdev vendor driver already
guarantees that a given mdev type is software compatible regardless of
the underlying hardware or driver version.  If it's not compatible in
these ways, different mdev types should be used.  If we then cross that
definition with migration compatibility then the mdev type changes
arbitrarily based on the version of the vendor driver in use.  How would
a user scripts accommodate that a kernel update changes the available
mdev types?  Also would such a scheme even resolve our problem, for
example are vendor drivers going to maintain compatibility with
previous versions in their latest driver?  Does a version imply that we
can only migrate to an identical version or does it imply any newer
version?
 
> BTW,
> 
> As far as I read the code, the migration check function would check 
> quite a lot of things before migration really happens, not only machine 
> type.
> 
> Mdev is listed as a sub-hierarchy of hostdev in the migration check 
> function. "hostdev" in the code means "a host device", like a 
> passthrough PCI device. The function would check the compatibility of 
> source device and destination device by types. e.g. for PCI passthrough 
> device, it would check the BDF.

Probably an example of how this code has never been used, matching BDF
between source and target is pretty much only relevant to the XML, it
has nothing to do with the compatibility of the device itself.

> For mdev, it doesn't check anything 
> right now. That's how this idea come out: Let libvirt have something to 
> check and know if the mdevs between source machine and destination 
> machine are compatible.
> 
> Simply knowing the type is not enough currently and we need prepare 
> something to let libvirt check the compatibility.
> 
> For how libvirt could check the compatibility of mdev, the above 
> investigation might be a hint.

It's good that there at least exists some framework for testing device
compatibility in libvirt, but we need to take it from the stub it seems
to be now for hostdev to something that actually provides some
reliability and robustness.  I'm also not sure if libvirt is the only
place we need to address this, QEMU itself should be able to attach
mdev defined meta data to the vmstate for a device.  I don't trust
vendor drivers enough to let them bury this inside their opaque device
state stream.  Thanks,

Alex