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

On Wed, 24 Apr 2019 04:15:58 -0400 Yan Zhao <yan.y.zhao(a)intel.com&gt; wrote: > On Wed, Apr 24, 2019 at 03:56:24PM +0800, Cornelia Huck wrote: > > On Tue, 23 Apr 2019 23:10:37 -0400 > > Yan Zhao <yan.y.zhao(a)intel.com&gt; wrote: > > > > > On Tue, Apr 23, 2019 at 05:59:32PM +0800, Cornelia Huck wrote: > > > > On Fri, 19 Apr 2019 04:35:04 -0400 > > > > Yan Zhao <yan.y.zhao(a)intel.com&gt; wrote: > > > > > @@ -225,6 +228,8 @@ Directories and files under the sysfs for Each Physical Device > > > > > [<type-id>], device_api, and available_instances are mandatory attributes > > > > > that should be provided by vendor driver. > > > > > > > > > > + version is a mandatory attribute if a mdev device supports live migration. > > > > > > > > What about "An mdev device wishing to support live migration must > > > > provide the version attribute."? > > > yes, I just want to keep consistent with the line above it > > > " [<type-id>], device_api, and available_instances are mandatory attributes > > > that should be provided by vendor driver." > > > what about below one? > > > "version is a mandatory attribute if a mdev device wishing to support live > > > migration." > > > > My point is that an attribute is not mandatory if it can be left out :) > > (I'm not a native speaker, though; maybe this makes perfect sense > > after all?) > > > > Maybe "version is a required attribute if live migration is supported > > for an mdev device"? > > > you are right, "mandatory" may bring some confusion. > Maybe > "vendor driver must provide version attribute for an mdev device wishing to > support live migration." ? > based on your first version :) "The vendor driver must provide the version attribute for any mdev device it wishes to support live migration for." ? > > > > > > > > > > > > + > > > > > * [<type-id>] > > > > > > > > > > The [<type-id>] name is created by adding the device driver string as a prefix > > > > > @@ -246,6 +251,35 @@ Directories and files under the sysfs for Each Physical Device > > > > > This attribute should show the number of devices of type <type-id> that can be > > > > > created. > > > > > > > > > > +* version > > > > > + > > > > > + This attribute is rw. It is used to check whether two devices are compatible > > > > > + for live migration. If this attribute is missing, then the corresponding mdev > > > > > + device is regarded as not supporting 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. > > > > > > > > I'm not sure whether a device that does not support live migration > > > > should expose this attribute in the first place. Or is that to cover > > > > cases where a driver supports live migration only for some of the > > > > devices it supports? > > > yes, driver returning error code is to cover the cases where only part of devices it > > > supports can be migrated. > > > > > > > > > > Also, I'm not sure if a string that has to be parsed is a good idea... > > > > is this 'version' attribute supposed to convey some human-readable > > > > information as well? The procedure you describe for compatibility > > > > checking does the checking within the vendor driver which I would > > > > expect to have a table/rules for that anyway. > > > right. if a vendor driver has the confidence to migrate between devices of > > > diffent platform or mdev types, it can maintain a compatibility table for that > > > purpose. That's the reason why we would leave the compatibility check to vendor > > > driver. vendor driver can freely choose its own complicated way to decide > > > which device is migratable to which device. > > > > I think there are two scenarios here: > > - Migrating between different device types, which is unlikely to work, > > except in special cases. > > - Migrating between different versions of the same device type, which > > may work for some drivers/devices (and at least migrating to a newer > > version looks quite reasonable). > > > > But both should be something that is decided by the individual driver; > > I hope we don't want to support migration between different drivers :-O > > > > Can we make this a driver-defined format? > > > yes, this is indeed driver-defined format. > Actually we define it into 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. > > vendor proprietary part is defined by vendor driver. vendor driver can > define any format it wishes to use. Also it is its own responsibility to > ensure backward compatibility if it wants to update format definition in this > part. > > So user space only needs to get source side's version string, and asks > target side whether the two are compatible. The decision maker is the > vendor driver:) If I followed the discussion correctly, I think you plan to drop this format, don't you? I'd be happy if a vendor driver can use a simple number without any prefixes if it so chooses. I also like the idea of renaming this "migration_version" so that it is clear we're dealing with versioning of the migration capability (and not a version of the device or so).

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