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

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 14 May 2019 03:47:36 -0400
Yan Zhao <yan.y.zhao@intel.com> wrote:

> On Tue, May 14, 2019 at 03:43:44PM +0800, Erik Skultety wrote:
> > On Tue, May 14, 2019 at 03:32:19AM -0400, Yan Zhao wrote:  
> > > On Tue, May 14, 2019 at 03:20:40PM +0800, Erik Skultety wrote:  

> > > > That said, from libvirt POV as a consumer, I'd expect there to be truly only 2
> > > > errors (I believe Alex has mentioned something similar in one of his responses
> > > > in one of the threads):
> > > >     a) read error indicating that an mdev type doesn't support migration
> > > >         - I assume if one type doesn't support migration, none of the other
> > > >           types exposed on the parent device do, is that a fair assumption?

Probably; but there might be cases where the migratability depends not
on the device type, but how the partitioning has been done... or is
that too contrived?

> > > >     b) write error indicating that the mdev types are incompatible for
> > > >     migration
> > > >
> > > > Regards,
> > > > Erik  
> > > Thanks for this explanation.
> > > so, can we arrive at below agreements?
> > >
> > > 1. "not to define the specific errno returned for a specific situation,
> > > let the vendor driver decide, userspace simply needs to know that an errno on
> > > read indicates the device does not support migration version comparison and
> > > that an errno on write indicates the devices are incompatible or the target
> > > doesn't support migration versions. "
> > > 2. vendor driver should log detailed error reasons in kernel log.  
> > 
> > That would be my take on this, yes, but I open to hear any other suggestions and
> > ideas I couldn't think of as well.

So, read to find out whether migration is supported at all, write to
find out whether it is supported for that concrete pairing is
reasonable for libvirt?

> > 
> > Erik  
> got it. thanks a lot!
> 
> hi Cornelia and Dave,
> do you also agree on:
> 1. "not to define the specific errno returned for a specific situation,
> let the vendor driver decide, userspace simply needs to know that an errno on
> read indicates the device does not support migration version comparison and
> that an errno on write indicates the devices are incompatible or the target
> doesn't support migration versions. "
> 2. vendor driver should log detailed error reasons in kernel log.

Two questions:
- How reasonable is it to refer to the system log in order to find out
  what exactly went wrong?
- If detailed error reporting is basically done to the syslog, do
  different error codes still provide useful information? Or should the
  vendor driver decide what it wants to do?