Re: [bug report] iommu/arm-smmu-v3: Event cannot be printed in some scenarios

Will Deacon
2024-07-24T10:24:23Z

On Wed, Jul 24, 2024 at 05:22:59PM +0800, Kunkun Jiang wrote:
> On 2024/7/24 9:42, Kunkun Jiang wrote:
> > drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> > 1797                 while (!queue_remove_raw(q, evt)) {
> > 1798                         u8 id = FIELD_GET(EVTQ_0_ID, evt[0]);
> > 1799
> > 1800                         ret = arm_smmu_handle_evt(smmu, evt);
> > 1801                         if (!ret || !__ratelimit(&rs))
> > 1802                                 continue;
> > 1803
> > 1804                         dev_info(smmu->dev, "event 0x%02x
> > received:\n", id);
> > 1805                         for (i = 0; i < ARRAY_SIZE(evt); ++i)
> > 1806                                 dev_info(smmu->dev, "\t0x%016llx\n",
> > 1807                                          (unsigned long
> > long)evt[i]);
> > 1808
> > 1809                         cond_resched();
> > 1810                 }
> >
> > The smmu-v3 driver cannot print event information when "ret" is 0.
> > Unfortunately due to commit 3dfa64aecbaf
> > ("iommu: Make iommu_report_device_fault() return void"), the default
> > return value in arm_smmu_handle_evt() is 0. Maybe a trace should
> > be added here?
>
> Additional explanation. Background introduction:
> 1.A device(VF) is passthrough(VFIO-PCI) to a VM.
> 2.The SMMU has the stall feature.
> 3.Modified guest device driver to generate an event.
>
> This event handling process is as follows:
> arm_smmu_evtq_thread
>     ret = arm_smmu_handle_evt
>         iommu_report_device_fault
>             iopf_param = iopf_get_dev_fault_param(dev);
>             // iopf is not enabled.
> // No RESUME will be sent!
>             if (WARN_ON(!iopf_param))
>                 return;
>     if (!ret || !__ratelimit(&rs))
>         continue;
>
> In this scenario, the io page-fault capability is not enabled.
> There are two problems here:
> 1. The event information is not printed.
> 2. The entire device(PF level) is stalled,not just the current
> VF. This affects other normal VFs.

Oh, so that stall is probably also due to b554e396e51c ("iommu: Make
iopf_group_response() return void"). I agree that we need a way to
propagate error handling back to the driver in the case that
'iopf_param' is NULL, otherwise we're making the unexpected fault
considerably more problematic than it needs to be.

Lu -- can we add the -ENODEV return back in the case that
iommu_report_device_fault() doesn't even find a 'iommu_fault_param' for
the device?

Will