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). -- libvir-list mailing list libvir-list(a)redhat.com https://www.redhat.com/mailman/listinfo/libvir-list