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

[Dr. David Alan Gilbert]

* Cornelia Huck (cohuck@xxxxxxxxxx) wrote:
> On Tue, 14 May 2019 03:47:36 -0400
> Yan Zhao <yan.y.zhao@xxxxxxxxx> 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?

I don't see error codes as being that helpful; if we can't actually get
an error message back up the stack (which was my preference), then I guess
syslog is as good as it will get.

Dave

--
Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK