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SubjectRe: [PATCH 1/2] vfio/mdev: add version field as mandatory attribute for mdev device
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@intel.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@intel.com> 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.
> > >
> > > It might make more sense if the driver does not register the attribute
> > > for the device in that case at all.
> > >
> > yes. what about rephrase like this:
> > "
> > 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 has two choices:
> > 1. do not register this version attribute
> > 2. return -ENODEV on rw this version attribute
>
> "return -ENODEV when accessing the version attribute" ?
Yeah, this one is better, thanks :)

> > Choice 1 is preferred.
> > "
> >
> >
> > > > 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
> > > >
> > > > 3. if two mdev devices are compatible, user space software can start
> > > > live migration, and vice versa.
> > > >
> > > > Note: if a mdev device does not support live migration, it either does
> > > > not provide a version attribute, or always returns errno when its version
> > > > attribute is read/written.
> > > >
> > > > Cc: Alex Williamson <alex.williamson@redhat.com>
> > > > Cc: Erik Skultety <eskultet@redhat.com>
> > > > Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > > > Cc: Cornelia Huck <cohuck@redhat.com>
> > > > Cc: "Tian, Kevin" <kevin.tian@intel.com>
> > > > Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
> > > > Cc: "Wang, Zhi A" <zhi.a.wang@intel.com>
> > > > Cc: Neo Jia <cjia@nvidia.com>
> > > > Cc: Kirti Wankhede <kwankhede@nvidia.com>
> > > >
> > > > Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
> > > > ---
> > > > Documentation/vfio-mediated-device.txt | 36 ++++++++++++++++++++++++++
> > > > samples/vfio-mdev/mbochs.c | 17 ++++++++++++
> > > > samples/vfio-mdev/mdpy.c | 16 ++++++++++++
> > > > samples/vfio-mdev/mtty.c | 16 ++++++++++++
> > > > 4 files changed, 85 insertions(+)
> > > >
> > > > diff --git a/Documentation/vfio-mediated-device.txt b/Documentation/vfio-mediated-device.txt
> > > > index c3f69bcaf96e..bc28471c0667 100644
> > > > --- a/Documentation/vfio-mediated-device.txt
> > > > +++ b/Documentation/vfio-mediated-device.txt
> > > > @@ -202,6 +202,7 @@ Directories and files under the sysfs for Each Physical Device
> > > > | | |--- available_instances
> > > > | | |--- device_api
> > > > | | |--- description
> > > > + | | |--- version
> > > > | | |--- [devices]
> > > > | |--- [<type-id>]
> > > > | | |--- create
> > > > @@ -209,6 +210,7 @@ Directories and files under the sysfs for Each Physical Device
> > > > | | |--- available_instances
> > > > | | |--- device_api
> > > > | | |--- description
> > > > + | | |--- version
> > > > | | |--- [devices]
> > > > | |--- [<type-id>]
> > > > | |--- create
> > > > @@ -216,6 +218,7 @@ Directories and files under the sysfs for Each Physical Device
> > > > | |--- available_instances
> > > > | |--- device_api
> > > > | |--- description
> > > > + | |--- version
> > > > | |--- [devices]
> > > >
> > > > * [mdev_supported_types]
> > > > @@ -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 :)

> >
> >
> > > > +
> > > > * [<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:)


> >
> > > I think you should also specify which errno writing an incompatible id
> > > is supposed to return (probably best something different than if the
> > > device does not support live migration at all, if we stick with
> > > creating the attribute in that case.)
> > Agree. I'll define it clearly in next revison.
>
> Thanks!

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