Re: [libvirt] [Qemu-devel] [PATCH 1/2] vfio/mdev: add version field as mandatory attribute for mdev device

On Fri, Apr 19, 2019 at 04:35:04AM -0400, Yan Zhao wrote: > device version attribute in mdev sysfs is used by user space software > (e.g. libvirt) to query device compatibility for live migration of VFIO > mdev devices. This attribute is mandatory if a mdev device supports live > migration. > > It consists of two parts: common part and vendor proprietary part. > common part: 32 bit. lower 16 bits is vendor id and higher 16 bits > identifies device type. e.g., for pci device, it is > "pci vendor id" | (VFIO_DEVICE_FLAGS_PCI << 16). > vendor proprietary part: this part is varied in length. vendor driver can > specify any string to identify a device. > > When reading this attribute, it should show device version string of the > device of type <type-id>. If a device does not support live migration, it > should return errno. > When writing a string to this attribute, it returns errno for > incompatibility or returns written string length in compatibility case. > If a device does not support live migration, it always returns errno. > > For user space software to use: > 1. > Before starting live migration, user space software first reads source side > mdev device's version. e.g. > "#cat \ > /sys/bus/pci/devices/0000\:00\:02.0/5ac1fb20-2bbf-4842-bb7e-36c58c3be9cd/mdev_type/version" > 00028086-193b-i915-GVTg_V5_4 > > 2. > Then, user space software writes the source side returned version string > to device version attribute in target side, and checks the return value. > If a negative errno is returned in the target side, then mdev devices in > source and target sides are not compatible; > If a positive number is returned and it equals to the length of written > string, then the two mdev devices in source and target side are compatible. > e.g. > (a) compatibility case > "# echo 00028086-193b-i915-GVTg_V5_4 > > /sys/bus/pci/devices/0000\:00\:02.0/882cc4da-dede-11e7-9180-078a62063ab1/mdev_type/version" > > (b) incompatibility case > "#echo 00028086-193b-i915-GVTg_V5_1 > > /sys/bus/pci/devices/0000\:00\:02.0/882cc4da-dede-11e7-9180-078a62063ab1/mdev_type/version" > -bash: echo: write error: Invalid argument What you have written here seems to imply that each mdev type is able to support many different versions at the same time. Writing a version into this sysfs file then chooses which of the many versions to actually use. This is good as it allows for live migration across driver software upgrades. A mgmt application may well want to know what versions are supported for an mdev type *before* starting a migration. A mgmt app can query all the 100's of hosts it knows and thus figure out which are valid to use as the target of a migration. IOW, we want to avoid the ever hitting the incompatibility case in the first place, by only choosing to migrate to a host that we know is going to be compatible. This would need some kind of way to report the full list of supported versions against the mdev supported types on the host.