Re: [PATCH 2/2] KVM: Use pci_store/load_saved_state() around VM device usage

Alex Williamson
2011-04-20T15:13:16Z

On Wed, 2011-04-20 at 10:19 +0300, Avi Kivity wrote:
> On 04/18/2011 10:43 PM, Alex Williamson wrote:
> > On Sun, 2011-04-17 at 12:25 +0300, Avi Kivity wrote:
> > > On 04/15/2011 10:54 PM, Alex Williamson wrote:
> > > > Store the device saved state so that we can reload the device back
> > > > to the original state when it's unassigned. This has the benefit
> > > > that the state survives across pci_reset_function() calls via
> > > > the PCI sysfs reset interface while the VM is using the device.
> > >
> > > > @@ -516,7 +518,7 @@ static int kvm_vm_ioctl_assign_device(struct kvm *kvm,
> > > >
> > > > pci_reset_function(dev);
> > > > pci_save_state(dev);
> > > > -
> > > > + match->pci_saved_state = pci_store_saved_state(dev);
> > > > match->assigned_dev_id = assigned_dev->assigned_dev_id;
> > >
> > > Error check?
> > >
> > > It might be better to give up the opacity of the data structure and make
> > > pci_saved_state the full struct, not a pointer.
> >
> > pci_store_saved_state() returns NULL on error, which is correctly
> > handled if we pass NULL to pci_load_saved_state() or a pointer to NULL
> > to pci_load_and_free_saved_state().
>
> But we silently swallow an error, this isn't good.
>
> > This is also why I changed the
> > __pci_reset_function() back to a normal pci_reset_function(), so we're
> > never left with an uninitialized device like we are now.
> >
> > We could be more verbose or return an error here, but we've gone for a
> > long time not even doing this save/restore across VM usage, so I don't
> > think it's worthy of preventing the device attachment if it fails.
>
> At least a log?

Ok, I'm not sure what corrective action a user would take or what they
should expect not to work, but I guess a KERN_DEBUG printk is
reasonable.

> Note avoiding the pointer would have removed the problem altogether.

Returning a struct on store? We lose any kind of opacity that way since
the caller needs to know about the struct then. I thought the pointer
makes it clear the caller shouldn't be touching the contents, but if you
think it's a better way to go, I can try it. Thanks,

Alex