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

[Janusz Krzysztofik]

On Wednesday, August 28, 2019 2:56:18 AM CEST Lu Baolu wrote:
> Hi Janusz,
> 
> On 8/27/19 5:35 PM, Janusz Krzysztofik wrote:
> > Hi Lu,
> > 
> > On Monday, August 26, 2019 10:29:12 AM CEST Lu Baolu wrote:
> >> Hi Janusz,
> >>
> >> On 8/26/19 4:15 PM, Janusz Krzysztofik wrote:
> >>> Hi Lu,
> >>>
> >>> On Friday, August 23, 2019 3:51:11 AM CEST Lu Baolu wrote:
> >>>> Hi,
> >>>>
> >>>> On 8/22/19 10:29 PM, Janusz Krzysztofik wrote:
> >>>>> When a perfectly working i915 device is hot unplugged (via sysfs) and
> >>>>> hot re-plugged again, its dev->archdata.iommu field is not populated
> >>>>> again with an IOMMU pointer.  As a result, the device probe fails on
> >>>>> DMA mapping error during scratch page setup.
> >>>>>
> >>>>> It looks like that happens because devices are not detached from their
> >>>>> MMUIO bus before they are removed on device unplug.  Then, when an
> >>>>> already registered device/IOMMU association is identified by the
> >>>>> reinstantiated device's bus and function IDs on IOMMU bus re-attach
> >>>>> attempt, the device's archdata is not populated with IOMMU information
> >>>>> and the bad happens.
> >>>>>
> >>>>> I'm not sure if this is a proper fix but it works for me so at least 
it
> >>>>> confirms correctness of my analysis results, I believe.  So far I
> >>>>> haven't been able to identify a good place where the possibly missing
> >>>>> IOMMU bus detach on device unplug operation could be added.
> >>>>
> >>>> Which kernel version are you testing with? Does it contain below 
commit?
> >>>>
> >>>> commit 458b7c8e0dde12d140e3472b80919cbb9ae793f4
> >>>> Author: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> >>>> Date:   Thu Aug 1 11:14:58 2019 +0800
> >>>
> >>> I was using an internal branch based on drm-tip which didn't contain 
this
> >>> commit yet.  Fortunately it has been already merged into drm-tip over 
last
> >>> weekend and has effectively fixed the issue.
> >>
> >> Thanks for testing this.
> > 
> > My testing appeared not sufficiently exhaustive. The fix indeed resolved 
my
> > initially discovered issue of not being able to rebind the i915 driver to 
a
> > re-plugged device, however it brought another, probably more serious 
problem
> > to light.
> > 
> > When an open i915 device is hot unplugged, IOMMU bus notifier now cleans 
up
> > IOMMU info for the device on PCI device remove while the i915 driver is 
still
> > not released, kept by open file descriptors.  Then, on last device close,
> > cleanup attempts lead to kernel panic raised from intel_unmap() on 
unresolved
> > IOMMU domain.
> 
> 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?

Thanks,
Janusz


> Best regards,
> Baolu
> 
> > 
> > With commit 458b7c8e0dde reverted and my fix applied, both late device 
close
> > and device re-plug work for me.  However, I can realize that's probably 
still
> > not a complete solution, possibly missing some protection against reuse of 
a
> > removed device other than for cleanup.  If you think that's the right way 
to
> > go, I can work more on that.
> > 
> > I've had a look at other drivers and found AMD is using somehow similar
> > approach.  On the other hand, looking at the IOMMU common code I couldn't
> > identify any arrangement that would support deferred device cleanup.
> > 
> > If that approach is not acceptable for Intel IOMMU, please suggest a way 
you'd
> > like to have it resolved and I can try to implement it.
> > 
> > Thanks,
> > Janusz
> > 
> >> Best regards,
> >> Lu Baolu
> >>
>