Re: [RFC PATCH] iommu/vt-d: Fix IOMMU field not populated on device hot re-plug

[Lu Baolu]

Hi,

On 8/29/19 3:58 PM, Janusz Krzysztofik wrote:
> Hi Baolu,
>
> On Thursday, August 29, 2019 3:43:31 AM CEST Lu Baolu wrote:
> > Hi Janusz,
> >
> > On 8/28/19 10:17 PM, Janusz Krzysztofik wrote:
> > > > We should avoid kernel panic when a intel_unmap() is called against
> > > > a non-existent domain.
> > > Does that mean you suggest to replace
> > > 	BUG_ON(!domain);
> > > with something like
> > > 	if (WARN_ON(!domain))
> > > 		return;
> > > and to not care of orphaned mappings left allocated?  Is there a way to
> inform
> > > users that their active DMA mappings are no longer valid and they
> shouldn't
> > > call dma_unmap_*()?
> > >
> > > > But we shouldn't expect the IOMMU driver not
> > > > cleaning up the domain info when a device remove notification comes and
> > > > wait until all file descriptors being closed, right?
> > > Shouldn't then the IOMMU driver take care of cleaning up resources still
> > > allocated on device remove before it invalidates and forgets their
> pointers?
> > >
> >
> > You are right. We need to wait until all allocated resources (iova and
> > mappings) to be released.
> >
> > How about registering a callback for BUS_NOTIFY_UNBOUND_DRIVER, and
> > removing the domain info when the driver detachment completes?
>
> Device core calls BUS_NOTIFY_UNBOUND_DRIVER on each driver unbind, regardless
> of a device being removed or not.  As long as the device is not unplugged and
> the BUS_NOTIFY_REMOVED_DEVICE notification not generated, an unbound driver is
> not a problem here.
> Morever, BUS_NOTIFY_UNBOUND_DRIVER  is called even before
> BUS_NOTIFY_REMOVED_DEVICE so that wouldn't help anyway.
> Last but not least, bus events are independent of the IOMMU driver use via
> DMA-API it exposes.

Fair enough.

> If keeping data for unplugged devices and reusing it on device re-plug is not
> acceptable then maybe the IOMMU driver should perform reference counting of
> its internal resources occupied by DMA-API users and perform cleanups on last
> release?

I am not saying that keeping data is not acceptable. I just want to
check whether there are any other solutions.

Best regards,
Baolu