Date:   Tue, 7 May 2019 01:39:13 -0400
From:   Yan Zhao <yan.y.zhao@...el.com>
To:     Cornelia Huck <cohuck@...hat.com>
Cc:     "cjia@...dia.com" <cjia@...dia.com>,
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Subject: Re: [PATCH 1/2] vfio/mdev: add version field as mandatory attribute
 for mdev device

On Tue, Apr 30, 2019 at 11:29:08PM +0800, Cornelia Huck wrote:
> On Wed, 24 Apr 2019 04:15:58 -0400
> Yan Zhao <yan.y.zhao@...el.com> 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@...el.com> 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@...el.com> 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).
hi Cornelia,
sorry I just saw this mail after sending v2 of this patch set...
yes, I dropped the common part and vendor driver now can define whatever it
wishes to identify a device version.
However, I don't agree to rename it to "migration_version", as it still may
bring some kind of confusing with the migration version a vendor driver is
using, e.g. vendor driver changes migration code and increases that migration
version.
In fact, what info we want to get from this attribute is whether this mdev
device is compatible with another mdev device, which is tied to device, and not
necessarily bound to migration.

do you think so?

Thanks
Yan
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