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Date:   Wed, 24 Apr 2019 09:56:24 +0200
From:   Cornelia Huck <cohuck@redhat.com>
To:     Yan Zhao <yan.y.zhao@intel.com>
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Subject: Re: [PATCH 1/2] vfio/mdev: add version field as mandatory attribute
 for mdev device
Message-ID: <20190424095624.0ce97328.cohuck@redhat.com>
In-Reply-To: <20190424031036.GB26247@joy-OptiPlex-7040>
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        <20190419083505.19654-1-yan.y.zhao@intel.com>
        <20190423115932.42619422.cohuck@redhat.com>
        <20190424031036.GB26247@joy-OptiPlex-7040>
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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" ?

> 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"?

> 
> 
> > > +
> > >  * [<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?

> 
> > 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!