Re: [RFC PATCH] s390: vfio-ap: Register the vfio_ap module for the "ap" parent bus

[Tony Krowiak <akrowiak@linux.ibm.com>]

On 12/15/21 18:02, Halil Pasic wrote:
> On Wed, 15 Dec 2021 13:51:02 +0100
> Cornelia Huck <cohuck@redhat.com> wrote:
>
>> On Wed, Dec 15 2021, Thomas Huth <thuth@redhat.com> wrote:
>>
>>> On 14/12/2021 22.55, Tony Krowiak wrote:
>>>>
>>>> On 12/13/21 11:11, Cornelia Huck wrote:
>>>>> One possibility is simply blocking autoload of the module in userspace by
>>>>> default, and only allow it to be loaded automatically when e.g. qemu-kvm
>>>>> is installed on the system. This is obviously something that needs to be
>>>>> decided by the distros.
>>>>>
>>>>> (kvm might actually be autoloaded already, so autoloading vfio-ap would
>>>>> not really make it worse.)
>>>> Of the vfio_ccw module is automatically loaded, then the kvm
>>>> module will also get loaded. I startup up a RHEL8.3 system and
>>>> sure enough, the vfio_ccw module is loaded along with the
>>>> kvm, vfio and mdev modules. If this is true for all distros, then
>>>> it wouldn't make much difference if the vfio_ap module is
>>>> autoloaded too.
>>> I think I don't mind too much if we auto-load vfio-ap or not - but I think
>>> we should make it consistent with vfio-ccw. So either auto-load both modules
>>> (if the corresponding devices are available), or remove the
>>> MODULE_DEVICE_TABLE() entries from both modules?
>> I think we really need to take a step back and think about the purpose
>> of adding a MODULE_DEVICE_TABLE()... basically, it declares which types
>> of devices on a certain bus a driver supports, in a way that can be
>> consumed by userspace (after file2alias.c worked on it).
> I did a quick search to locate where this semantic was codified. But
> I didn't find the place neither Documentation/ nor in the header where
> MODULE_DEVICE_TABLE is defined.
>
>> Userspace typically uses this to match devices it is notified about to
>> drivers that could possibly drive those devices. In general, the
>> assumption is that you will want to have the drivers for your devices
>> loaded. In some cases (drivers only used in special cases, like here),
>> it might be a better idea to autoload the drivers only under certain
>> circumstances (e.g. if you know you're going to run KVM guests).
> Does RHEL do this, or would RHEL do this out of the box? I.e.
> would we end up preserving old behavior when this fix hits the distro,
> or would the end user end up with kvm and vfio_ap loaded (out of the
> box)?
>
> What would be the mechanism of choice to implement if kvm loaded and
> APs present/hotplugged load vfio_ap, otherwise don't in the userspace?
>
> Sorry I'm not very familiar with this whole modules auto-loading
> business, so I may be asking obvious questions. But a quick google
> search did not help me.
>
>> My main point, however, is that we're talking about policy here: whether
>> a potentially useful driver should be loaded or not is a decision that
>> should be made by userspace. Not providing a MODULE_DEVICE_TABLE does
>> not look like the right solution, as it deprives userspace of the
>> information to autoload the driver, if it actually wants to do so.
>>
> I'm sympathetic to this reading of the situation, but I'm not sure
> it is as black and white as stated.
>
> I think the current state of affairs in the vfio_ap module is clearly a
> bug.
>
> One can argue that not auto-loading vfio_ap and kvm per default out of
> the box is not a bug. I mean the tooling (chzdev) seems to do fine
> without this and just assuming that both kvm and vfio_ap will be needed
> just because we have APs seems wrong.
>
> One of the more important guiding principles of Linux kernel development
> is no userspace regressions. And I think suddenly getting vfio_ap and kvm
> loaded just because we have AP devices can be thought of as a regression.
>
> So I'm sympathetic to Harald's view as well.
>
> Of course there is the solution that the distros should really make sure
> the old behavior is preserved, or some smart behavior is introduced. But
> regarding smart, I believe "if you have devices that are configured for
> vfio_ap pass-through (with chzdev), then the vfio_ap module should get
> loaded" is pretty much as smart as it gets. So blacklisting the module
> by default in the distros looks like a viable option. If that is what
> we want, we should probably ask the distros, because I don't think
> it is just obviously their job to figure out that they have to do so.
>
> Disclaimer: I might be wrong about the current behavior, I didn't verify
> my claims
>
> Also what does vfio-pci do?

 From vfio_pci.c:

static const struct pci_device_id vfio_pci_table[] = {
     { PCI_DRIVER_OVERRIDE_DEVICE_VFIO(PCI_ANY_ID, PCI_ANY_ID) }, /* 
match all by default */
     {}
};

MODULE_DEVICE_TABLE(pci, vfio_pci_table);

> As far as I can tell vfio-pci does not
> participate in module auto loading just because there are pci devices.
> The have some smart override I don't quite understand:
> https://patchwork.ozlabs.org/project/linux-pci/patch/20210826103912.128972-11-yishaih@nvidia.com/
> Before, I don't think they had a MODULE_DEVICE_TABLE:
> https://elixir.bootlin.com/linux/v5.8.18/source/drivers/vfio/pci/vfio_pci.c
>
> Regards,
> Halil