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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Cornelia Huck (cohuck@redhat.com) wrote:
> On Thu, 9 May 2019 17:48:26 +0100
> "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> 
> > * Cornelia Huck (cohuck@redhat.com) wrote:
> > > On Thu, 9 May 2019 16:48:57 +0100
> > > "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> > >   
> > > > * Cornelia Huck (cohuck@redhat.com) wrote:  
> > > > > On Tue, 7 May 2019 15:18:26 -0600
> > > > > Alex Williamson <alex.williamson@redhat.com> wrote:
> > > > >     
> > > > > > On Sun,  5 May 2019 21:49:04 -0400
> > > > > > Yan Zhao <yan.y.zhao@intel.com> wrote:    
> > > > >     
> > > > > > > +  Errno:
> > > > > > > +  If vendor driver wants to claim a mdev device incompatible to all other mdev
> > > > > > > +  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;      
> > > > > > 
> > > > > > I think it's best 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.    
> > > > > 
> > > > > I think I have to disagree here: It's probably valuable to have an
> > > > > agreed error for 'cannot migrate at all' vs 'cannot migrate between
> > > > > those two particular devices'. Userspace might want to do different
> > > > > things (e.g. trying with different device pairs).    
> > > > 
> > > > Trying to stuff these things down an errno seems a bad idea; we can't
> > > > get much information that way.  
> > > 
> > > So, what would be a reasonable approach? Userspace should first read
> > > the version attributes on both devices (to find out whether migration
> > > is supported at all), and only then figure out via writing whether they
> > > are compatible?
> > > 
> > > (Or just go ahead and try, if it does not care about the reason.)  
> > 
> > Well, I'm OK with something like writing to test whether it's
> > compatible, it's just we need a better way of saying 'no'.
> > I'm not sure if that involves reading back from somewhere after
> > the write or what.
> 
> Hm, so I basically see two ways of doing that:
> - standardize on some error codes... problem: error codes can be hard
>   to fit to reasons
> - make the error available in some attribute that can be read
> 
> I'm not sure how we can serialize the readback with the last write,
> though (this looks inherently racy).
> 
> How important is detailed error reporting here?

I think we need something, otherwise we're just going to get vague
user reports of 'but my VM doesn't migrate'; I'd like the error to be
good enough to point most users to something they can understand
(e.g. wrong card family/too old a driver etc).

Dave

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