Re: [PATCH v3 09/14] iommu/arm-smmu: Prevent forced unbinding of Arm SMMU drivers

[John Garry]

On 26/11/2019 20:27, Saravana Kannan wrote:
> On Tue, Nov 26, 2019 at 1:13 AM John Garry <john.garry@xxxxxxxxxx> wrote:
> > On 21/11/2019 11:49, Will Deacon wrote:
> > > Forcefully unbinding the Arm SMMU drivers is a pretty dangerous operation,
> > > since it will likely lead to catastrophic failure for any DMA devices
> > > mastering through the SMMU being unbound. When the driver then attempts
> > > to "handle" the fatal faults, it's very easy to trip over dead data
> > > structures, leading to use-after-free.
> > >
> > > On John's machine, he reports that the machine was "unusable" due to
> > > loss of the storage controller following a forced unbind of the SMMUv3
> > > driver:
> > >
> > >     | # cd ./bus/platform/drivers/arm-smmu-v3
> > >     | # echo arm-smmu-v3.0.auto > unbind
> > >     | hisi_sas_v2_hw HISI0162:01: CQE_AXI_W_ERR (0x800) found!
> > >     | platform arm-smmu-v3.0.auto: CMD_SYNC timeout at 0x00000146
> > >     | [hwprod 0x00000146, hwcons 0x00000000]
> > >
> > > Prevent this forced unbinding of the drivers by setting "suppress_bind_attrs"
> > > to true.
> >
> > This seems a reasonable approach for now.
> >
> > BTW, I'll give this series a spin this week, which again looks to be
> > your iommu/module branch, excluding the new IORT patch.

Hi Saravana,

> Is this on a platform where of_devlink creates device links between
> the iommu device and its suppliers?I'm guessing no? Because device
> links should for unbinding of all the consumers before unbinding the
> supplier.

I'm only really interested in ACPI, TBH.

> Looks like it'll still allow the supplier to unbind if the consumers
> don't allow unbinding. Is that the case here?

So just unbinding the driver from a device does not delete the device 
nor exit the device from it's IOMMU group - so we keep the reference to 
the SMMU ko. As such, I don't know how to realistically test unloading 
the SMMU ko when we have platform devices involved. Maybe someone can 
enlighten me...

Thanks,
John