Re: [PATCH 0/5] QEMU VFIO live migration

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 28 Mar 2019 10:21:38 +0100
Erik Skultety <eskultet@redhat.com> wrote:

> On Thu, Mar 28, 2019 at 04:36:03AM -0400, Zhao Yan wrote:
> > hi Alex and Dave,
> > Thanks for your replies.
> > Please see my comments inline.
> >
> > On Thu, Mar 28, 2019 at 06:10:20AM +0800, Alex Williamson wrote:  
> > > On Wed, 27 Mar 2019 20:18:54 +0000
> > > "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> > >  
> > > > * Zhao Yan (yan.y.zhao@intel.com) wrote:  
> > > > > On Wed, Feb 20, 2019 at 07:42:42PM +0800, Cornelia Huck wrote:  
> > > > > > > > > >   b) How do we detect if we're migrating from/to the wrong device or
> > > > > > > > > > version of device?  Or say to a device with older firmware or perhaps
> > > > > > > > > > a device that has less device memory ?  
> > > > > > > > > Actually it's still an open for VFIO migration. Need to think about
> > > > > > > > > whether it's better to check that in libvirt or qemu (like a device magic
> > > > > > > > > along with verion ?).  
> > > > > > >
> > > > > > > We must keep the hardware generation is the same with one POD of public cloud
> > > > > > > providers. But we still think about the live migration between from the the lower
> > > > > > > generation of hardware migrated to the higher generation.  
> > > > > >
> > > > > > Agreed, lower->higher is the one direction that might make sense to
> > > > > > support.
> > > > > >
> > > > > > But regardless of that, I think we need to make sure that incompatible
> > > > > > devices/versions fail directly instead of failing in a subtle, hard to
> > > > > > debug way. Might be useful to do some initial sanity checks in libvirt
> > > > > > as well.
> > > > > >
> > > > > > How easy is it to obtain that information in a form that can be
> > > > > > consumed by higher layers? Can we find out the device type at least?
> > > > > > What about some kind of revision?  
> > > > > hi Alex and Cornelia
> > > > > for device compatibility, do you think it's a good idea to use "version"
> > > > > and "device version" fields?
> > > > >
> > > > > version field: identify live migration interface's version. it can have a
> > > > > sort of backward compatibility, like target machine's version >= source
> > > > > machine's version. something like that.  
> > >
> > > Don't we essentially already have this via the device specific region?
> > > The struct vfio_info_cap_header includes id and version fields, so we
> > > can declare a migration id and increment the version for any
> > > incompatible changes to the protocol.  
> > yes, good idea!
> > so, what about declaring below new cap?
> >     #define VFIO_REGION_INFO_CAP_MIGRATION 4
> >     struct vfio_region_info_cap_migration {
> >         struct vfio_info_cap_header header;
> >         __u32 device_version_len;
> >         __u8  device_version[];
> >     };

I'm not sure why we need a new region for everything, it seems this
could fit within the protocol of a single region.  This could simply be
a new action to retrieve the version where the protocol would report
the number of bytes available, just like the migration stream itself.

> > > > > device_version field consists two parts:
> > > > > 1. vendor id : it takes 32 bits. e.g. 0x8086.  
> > >
> > > Who allocates IDs?  If we're going to use PCI vendor IDs, then I'd
> > > suggest we use a bit to flag it as such so we can reserve that portion
> > > of the 32bit address space.  See for example:
> > >
> > > #define VFIO_REGION_TYPE_PCI_VENDOR_TYPE        (1 << 31)
> > > #define VFIO_REGION_TYPE_PCI_VENDOR_MASK        (0xffff)
> > >
> > > For vendor specific regions.  
> > Yes, use PCI vendor ID.
> > you are right, we need to use highest bit (VFIO_REGION_TYPE_PCI_VENDOR_TYPE)
> > to identify it's a PCI ID.
> > Thanks for pointing it out.
> > But, I have a question. what is VFIO_REGION_TYPE_PCI_VENDOR_MASK used for?
> > why it's 0xffff? I searched QEMU and kernel code and did not find anywhere
> > uses it.

PCI vendor IDs are 16bits, it's just indicating that when the
PCI_VENDOR_TYPE bit is set the valid data is the lower 16bits.

> > > > > 2. vendor proprietary string: it can be any string that a vendor driver
> > > > > thinks can identify a source device. e.g. pciid + mdev type.
> > > > > "vendor id" is to avoid overlap of "vendor proprietary string".
> > > > >
> > > > >
> > > > > struct vfio_device_state_ctl {
> > > > >      __u32 version;            /* ro */
> > > > >      __u8 device_version[MAX_DEVICE_VERSION_LEN];            /* ro */
> > > > >      struct {
> > > > >      	__u32 action; /* GET_BUFFER, SET_BUFFER, IS_COMPATIBLE*/
> > > > > 	...
> > > > >      }data;
> > > > >      ...
> > > > >  };  
> > >
> > > We have a buffer area where we can read and write data from the vendor
> > > driver, why would we have this IS_COMPATIBLE protocol to write the
> > > device version string but use a static fixed length version string in
> > > the control header to read it?  IOW, let's use GET_VERSION,
> > > CHECK_VERSION actions that make use of the buffer area and allow vendor
> > > specific version information length.  
> > you are right, such static fixed length version string is bad :)
> > To get device version, do you think which approach below is better?
> > 1. use GET_VERSION action, and read from region buffer
> > 2. get it when querying cap VFIO_REGION_INFO_CAP_MIGRATION
> >
> > seems approach 1 is better, and cap VFIO_REGION_INFO_CAP_MIGRATION is only
> > for checking migration interface's version?

I think 1 provides the most flexibility to the vendor driver.

> > > > > Then, an action IS_COMPATIBLE is added to check device compatibility.
> > > > >
> > > > > The flow to figure out whether a source device is migratable to target device
> > > > > is like that:
> > > > > 1. in source side's .save_setup, save source device's device_version string
> > > > > 2. in target side's .load_state, load source device's device version string
> > > > > and write it to data region, and call IS_COMPATIBLE action to ask vendor driver
> > > > > to check whether the source device is compatible to it.
> > > > >
> > > > > The advantage of adding an IS_COMPATIBLE action is that, vendor driver can
> > > > > maintain a compatibility table and decide whether source device is compatible
> > > > > to target device according to its proprietary table.
> > > > > In device_version string, vendor driver only has to describe the source
> > > > > device as elaborately as possible and resorts to vendor driver in target side
> > > > > to figure out whether they are compatible.  
> > >
> > > I agree, it's too complicated and restrictive to try to create an
> > > interface for the user to determine compatibility, let the driver
> > > declare it compatible or not.  
> > :)
> >  
> > > > It would also be good if the 'IS_COMPATIBLE' was somehow callable
> > > > externally - so we could be able to answer a question like 'can we
> > > > migrate this VM to this host' - from the management layer before it
> > > > actually starts the migration.  
> >
> > so qemu needs to expose two qmp/sysfs interfaces: GET_VERSION and CHECK_VERSION.
> > GET_VERSION returns a vm's device's version string.
> > CHECK_VERSION's input is device version string and return
> > compatible/non-compatible.
> > Do you think it's good?

That's the idea, but note that QEMU can only provide the QMP interface,
the sysfs interface would of course be provided as more of a direct
path from the vendor driver or mdev kernel layer.

> > > I think we'd need to mirror this capability in sysfs to support that,
> > > or create a qmp interface through QEMU that the device owner could make
> > > the request on behalf of the management layer.  Getting access to the
> > > vfio device requires an iommu context that's already in use by the
> > > device owner, we have no intention of supporting a model that allows
> > > independent tasks access to a device.  Thanks,
> > > Alex
> > >  
> > do you think two sysfs nodes under a device node is ok?
> > e.g.
> > /sys/devices/pci0000\:00/0000\:00\:02.0/882cc4da-dede-11e7-9180-078a62063ab1/get_version
> > /sys/devices/pci0000\:00/0000\:00\:02.0/882cc4da-dede-11e7-9180-078a62063ab1/check_version  

I'd think it might live more in the mdev_support_types area, wouldn't
we ideally like to know if a device is compatible even before it's
created?  For example maybe:

/sys/class/mdev_bus/0000:00:02.0/mdev_supported_types/i915-GVTg_V5_4/version

Where reading the sysfs attribute returns the version string and
writing a string into the attribute return an errno for incompatibility.

> Why do you need both sysfs and QMP at the same time? I can see it gives us some
> flexibility, but is there something more to that?
>
> Normally, I'd prefer a QMP interface from libvirt's perspective (with an
> appropriate capability that libvirt can check for QEMU support) because I imagine large nodes having a
> bunch of GPUs with different revisions which might not be backwards compatible.
> Libvirt might query the version string on source and check it on dest via the
> QMP in a way that QEMU, talking to the driver, would return either a list or a
> single physical device to which we can migrate, because neither QEMU nor
> libvirt know that, only the driver does, so that's an important information
> rather than looping through all the devices and trying to find one that is
> compatible. However, you might have a hard time making all the necessary
> changes in QMP introspectable, a new command would be fine, but if you also
> wanted to extend say vfio-pci options, IIRC that would not appear in the QAPI
> schema and libvirt would not be able to detect support for it.
> 
> On the other hand, the presence of a QMP interface IMO doesn't help mgmt apps
> much, as it still carries the burden of being able to check this only at the
> time of migration, which e.g. OpenStack would like to know long before that.
> 
> So, having sysfs attributes would work for both libvirt (even though libvirt
> would benefit from a QMP much more) and OpenStack. OpenStack would IMO then
> have to figure out how to create the mappings between compatible devices across
> several nodes which are non-uniform.

Yep, vfio encompasses more than just QEMU, so a sysfs interface has more
utility than a QMP interface.  For instance we couldn't predetermine if
an mdev type on a host is compatible if we need to first create the
device and launch a QEMU instance on it to get access to QMP.  So maybe
the question is whether we should bother with any sort of VFIO API to
do this comparison, perhaps only a sysfs interface is sufficient for a
complete solution.  The downside of not having a version API in the
user interface might be that QEMU on its own can only try a migration
and see if it fails, it wouldn't have the ability to test expected
compatibility without access to sysfs.  And maybe that's fine.  Thanks,

Alex