Re: [Qemu-devel] [PATCH v2 1/2] vfio/mdev: add version attribute for mdev device

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 8 May 2019 07:57:05 -0400
Yan Zhao <yan.y.zhao@intel.com> wrote:

> On Tue, May 07, 2019 at 05:19:54PM +0800, Cornelia Huck wrote:
> > On Sun,  5 May 2019 21:49:04 -0400
> > Yan Zhao <yan.y.zhao@intel.com> wrote:
> >   
> > > version attribute is used to check two mdev devices' compatibility.
> > > 
> > > The key point of this version attribute is that it's rw.
> > > User space has no need to understand internal of device version and no
> > > need to compare versions by itself.
> > > Compared to reading version strings from both two mdev devices being
> > > checked, user space only reads from one mdev device's version attribute.
> > > After getting its version string, user space writes this string into the
> > > other mdev device's version attribute. Vendor driver of mdev device
> > > whose version attribute being written will check device compatibility of
> > > the two mdev devices for user space and return success for compatibility
> > > or errno for incompatibility.  
> > 
> > I'm still missing a bit _what_ is actually supposed to be
> > compatible/incompatible. I'd assume some internal state descriptions
> > (even if this is not actually limited to migration).
> >  
> right.
> originally, I thought this attribute should only contain a device's hardware
> compatibility info. But seems also including vendor specific software migration
> version is more reasonable, because general VFIO migration code cannot know
> version of vendor specific software migration code until migration data is
> transferring to the target vm. Then renaming it to migration_version is more
> appropriate.
> :)

Nod.

(...)

> > > @@ -246,6 +249,143 @@ 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, and is optional.
> > > +  It is used to check device compatibility between two mdev devices and is
> > > +  accessed in pairs between the two mdev devices being checked.
> > > +  The intent of this attribute is to make an mdev device's version opaque to
> > > +  user space, so instead of reading two mdev devices' version strings and
> > > +  comparing in userspace, user space should only read one mdev device's version
> > > +  attribute, and writes this version string into the other mdev device's version
> > > +  attribute. Then vendor driver of mdev device whose version attribute being
> > > +  written would check the incoming version string and tell user space whether
> > > +  the two mdev devices are compatible via return value. That's why this
> > > +  attribute is writable.  
> > 
> > I would reword this a bit:
> > 
> > "This attribute provides a way to check device compatibility between
> > two mdev devices from userspace. The intended usage is for userspace to
> > read the version attribute from one mdev device and then writing that
> > value to the version attribute of the other mdev device. The second
> > mdev device indicates compatibility via the return code of the write
> > operation. This makes compatibility between mdev devices completely
> > vendor-defined and opaque to userspace."
> > 
> > We still should explain _what_ compatibility we're talking about here,
> > though.
> >   
> Thanks. It's much better than mine:) 
> Then I'll change compatibility --> migration compatibility.

Ok, with that it should be clear enough.

> 
> > > +
> > > +  when reading this attribute, it should show device version string of
> > > +  the device of type <type-id>.
> > > +
> > > +  This string is private to vendor driver itself. Vendor driver is able to
> > > +  freely define format and length of device version string.
> > > +  e.g. It can use a combination of pciid of parent device + mdev type.
> > > +
> > > +  When writing a string to this attribute, vendor driver should analyze this
> > > +  string and check whether the mdev device being identified by this string is
> > > +  compatible with the mdev device for this attribute. vendor driver should then
> > > +  return written string's length if it regards the two mdev devices are
> > > +  compatible; vendor driver should return negative errno if it regards the two
> > > +  mdev devices are not compatible.
> > > +
> > > +  User space should treat ANY of below conditions as two mdev devices not
> > > +  compatible:
> > > +  (1) any one of the two mdev devices does not have a version attribute
> > > +  (2) error when read from one mdev device's version attribute  
> > 
> > s/read/reading/
> >   
> > > +  (3) error when write one mdev device's version string to the other mdev  
> > 
> > s/write/writing/
> >   
> > > +  device's version attribute
> > > +
> > > +  User space should regard two mdev devices compatible when ALL of below
> > > +  conditions are met:
> > > +  (1) success when read from one mdev device's version attribute.  
> > 
> > s/read/reading/
> >   
> > > +  (2) success when write one mdev device's version string to the other mdev  
> > 
> > s/write/writing/  
> got it. thanks for pointing them out:)
> >   
> > > +  device's version attribute
> > > +
> > > +  Errno:
> > > +  If vendor driver wants to claim a mdev device incompatible to all other mdev  
> > 
> > "If the vendor driver wants to designate a mdev device..."
> >   
> ok. thanks:)
> > > +  devices, it should not register version attribute for this mdev device. But if
> > > +  a vendor driver has already registered version attribute and it wants to claim
> > > +  a mdev device incompatible to all other mdev devices, it needs to return
> > > +  -ENODEV on access to this mdev device's version attribute.
> > > +  If a mdev device is only incompatible to certain mdev devices, write of
> > > +  incompatible mdev devices's version strings to its version attribute should
> > > +  return -EINVAL;  
> > 
> > 
> > Maybe put the defined return code into a bulleted list instead? But
> > this looks reasonable as well.
> >   
> as user space have no idea of those errno and only gets 0/1 as return code from
> read/write. maybe I can move this description of errno to patch 2/2 as an
> example?

Confused. They should get -EINVAL/-ENODEV/... all right, shouldn't they?

> 
> > > +
> > > +  This attribute can be taken advantage of by live migration.
> > > +  If user space detects two mdev devices are compatible through version
> > > +  attribute, it can start migration between the two mdev devices, otherwise it
> > > +  should abort its migration attempts between the two mdev devices.  
> > 
> > (...)
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