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

[Yan Zhao]

On Fri, May 10, 2019 at 05:48:38PM +0800, Cornelia Huck wrote:
> On Fri, 10 May 2019 10:36:09 +0100
> "Dr. David Alan Gilbert" <dgilbert@xxxxxxxxxx> wrote:
> 
> > * Cornelia Huck (cohuck@xxxxxxxxxx) wrote:
> > > On Thu, 9 May 2019 17:48:26 +0100
> > > "Dr. David Alan Gilbert" <dgilbert@xxxxxxxxxx> wrote:
> > >   
> > > > * Cornelia Huck (cohuck@xxxxxxxxxx) wrote:  
> > > > > On Thu, 9 May 2019 16:48:57 +0100
> > > > > "Dr. David Alan Gilbert" <dgilbert@xxxxxxxxxx> wrote:
> > > > >     
> > > > > > * Cornelia Huck (cohuck@xxxxxxxxxx) wrote:    
> > > > > > > On Tue, 7 May 2019 15:18:26 -0600
> > > > > > > Alex Williamson <alex.williamson@xxxxxxxxxx> wrote:
> > > > > > >       
> > > > > > > > On Sun,  5 May 2019 21:49:04 -0400
> > > > > > > > Yan Zhao <yan.y.zhao@xxxxxxxxx> 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).
> 
> Ok, that sounds like a reasonable point. Not that I have a better idea
> how to achieve that, though... we could also log a more verbose error
> message to the kernel log, but that's not necessarily where a user will
> look first.
> 
> Ideally, we'd want to have the user space program setting up things
> querying the general compatibility for migration (so that it becomes
> their problem on how to alert the user to problems :), but I'm not sure
> how to eliminate the race between asking the vendor driver for
> compatibility and getting the result of that operation.
> 
> Unless we introduce an interface that can retrieve _all_ results
> together with the written value? Or is that not going to be much of a
> problem in practice?
what about defining a migration_errors attribute, storing recent 10 error
records with format like:
    input string: error
as identical input strings always have the same error string, the 10 error
records may meet 10+ reason querying operations. And in practice, I think there
wouldn't be 10 simultaneous migration requests?

or could we just define some common errno? like 
#define ENOMIGRATION         140  /* device not supporting migration */
#define EUNATCH              49  /* software version not match */
#define EHWNM                142  /* hardware not matching*/