[libvirt] [RFC PATCH 1/9] vfio/pci: introduce mediate ops to intercept vfio-pci ops

[Alex Williamson]

On Mon, 9 Dec 2019 21:44:23 -0500
Yan Zhao <yan.y.zhao at intel.com> wrote:

> > > > > Currently, yes, i40e has build dependency on vfio-pci.
> > > > > It's like this, if i40e decides to support SRIOV and compiles in vf
> > > > > related code who depends on vfio-pci, it will also have build dependency
> > > > > on vfio-pci. isn't it natural?    
> > > > 
> > > > No, this is not natural.  There are certainly i40e VF use cases that
> > > > have no interest in vfio and having dependencies between the two
> > > > modules is unacceptable.  I think you probably want to modularize the
> > > > i40e vfio support code and then perhaps register a table in vfio-pci
> > > > that the vfio-pci code can perform a module request when using a
> > > > compatible device.  Just and idea, there might be better options.  I
> > > > will not accept a solution that requires unloading the i40e driver in
> > > > order to unload the vfio-pci driver.  It's inconvenient with just one
> > > > NIC driver, imagine how poorly that scales.
> > > >     
> > > what about this way:
> > > mediate driver registers a module notifier and every time when
> > > vfio_pci is loaded, register to vfio_pci its mediate ops?
> > > (Just like in below sample code)
> > > This way vfio-pci is free to unload and this registering only gives
> > > vfio-pci a name of what module to request.
> > > After that,
> > > in vfio_pci_open(), vfio-pci requests the mediate driver. (or puts
> > > the mediate driver when mediate driver does not support mediating the
> > > device)
> > > in vfio_pci_release(), vfio-pci puts the mediate driver.
> > > 
> > > static void register_mediate_ops(void)
> > > {
> > >         int (*func)(struct vfio_pci_mediate_ops *ops) = NULL;
> > > 
> > >         func = symbol_get(vfio_pci_register_mediate_ops);
> > > 
> > >         if (func) {
> > >                 func(&igd_dt_ops);
> > >                 symbol_put(vfio_pci_register_mediate_ops);
> > >         }
> > > }
> > > 
> > > static int igd_module_notify(struct notifier_block *self,
> > >                               unsigned long val, void *data)
> > > {
> > >         struct module *mod = data;
> > >         int ret = 0;
> > > 
> > >         switch (val) {
> > >         case MODULE_STATE_LIVE:
> > >                 if (!strcmp(mod->name, "vfio_pci"))
> > >                         register_mediate_ops();
> > >                 break;
> > >         case MODULE_STATE_GOING:
> > >                 break;
> > >         default:
> > >                 break;
> > >         }
> > >         return ret;
> > > }
> > > 
> > > static struct notifier_block igd_module_nb = {
> > >         .notifier_call = igd_module_notify,
> > >         .priority = 0,
> > > };
> > > 
> > > 
> > > 
> > > static int __init igd_dt_init(void)
> > > {
> > > 	...
> > > 	register_mediate_ops();
> > > 	register_module_notifier(&igd_module_nb);
> > > 	...
> > > 	return 0;
> > > }  
> > 
> > 
> > No, this is bad.  Please look at MODULE_ALIAS() and request_module() as
> > used in the vfio-platform for loading reset driver modules.  I think
> > the correct approach is that vfio-pci should perform a request_module()
> > based on the device being probed.  Having the mediation provider
> > listening for vfio-pci and registering itself regardless of whether we
> > intend to use it assumes that we will want to use it and assumes that
> > the mediation provider module is already loaded.  We should be able to
> > support demand loading of modules that may serve no other purpose than
> > providing this mediation.  Thanks,  
> hi Alex
> Thanks for this message.
> So is it good to create a separate module as mediation provider driver,
> and alias its module name to "vfio-pci-mediate-vid-did".
> Then when vfio-pci probes the device, it requests module of that name ?

I think this would give us an option to have the mediator as a separate
module, but not require it.  Maybe rather than a request_module(),
where if we follow the platform reset example we'd then expect the init
code for the module to register into a list, we could do a
symbol_request().  AIUI, this would give us a reference to the symbol
if the module providing it is already loaded, and request a module
(perhaps via an alias) if it's not already load.  Thanks,

Alex