Re: [PATCH RFC 10/11] iommu: Make IOPF handling framework generic

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

On Tue, Mar 22, 2022 at 10:24:26AM +0000, Tian, Kevin wrote:
> > From: Jean-Philippe Brucker <jean-philippe@linaro.org>
> > Sent: Tuesday, March 22, 2022 6:06 PM
> > 
> > On Tue, Mar 22, 2022 at 01:00:08AM +0000, Tian, Kevin wrote:
> > > > From: Jean-Philippe Brucker <jean-philippe@linaro.org>
> > > > Sent: Monday, March 21, 2022 7:42 PM
> > > >
> > > > Hi Kevin,
> > > >
> > > > On Mon, Mar 21, 2022 at 08:09:36AM +0000, Tian, Kevin wrote:
> > > > > > From: Lu Baolu <baolu.lu@linux.intel.com>
> > > > > > Sent: Sunday, March 20, 2022 2:40 PM
> > > > > >
> > > > > > The existing IOPF handling framework only handles the I/O page faults
> > for
> > > > > > SVA. Ginven that we are able to link iommu domain with each I/O
> > page
> > > > fault,
> > > > > > we can now make the I/O page fault handling framework more
> > general
> > > > for
> > > > > > more types of page faults.
> > > > >
> > > > > "make ... generic" in subject line is kind of confusing. Reading this patch
> > I
> > > > > think you really meant changing from per-device fault handling to per-
> > > > domain
> > > > > fault handling. This is more accurate in concept since the fault is caused
> > by
> > > > > the domain page table. 😊
> > > >
> > > > I tend to disagree with that last part. The fault is caused by a specific
> > > > device accessing shared page tables. We should keep that device
> > > > information throughout the fault handling, so that we can report it to the
> > > > driver when things go wrong. A process can have multiple threads bound
> > to
> > > > different devices, they share the same mm so if the driver wanted to
> > > > signal a misbehaving thread, similarly to a SEGV on the CPU side, it would
> > > > need the device information to precisely report it to userspace.
> > > >
> > >
> > > iommu driver can include the device information in the fault data. But
> > > in concept the IOPF should be reported per domain.
> > 
> > So I don't remember where we left off on that topic, what about fault
> > injection into guests?  In that case device info is more than just
> > diagnostic, fault injection can't work without it. I think we talked about
> > passing a device cookie to userspace, just want to make sure.
> > 
> > > and I agree with Jason that at most we can send SEGV to the entire thread
> > > group since there is no way to associate a DMA back to a thread which
> > > initiates the DMA.
> > 
> > The point is providing the most accurate information to the device driver
> > for diagnostics and debugging. A process opens multiple queues to
> > different devices, then if one of the queues issues invalid DMA, the
> > driver won't even know which queue is broken if you only report the target
> > mm and not the source dev. I don't think we gain anything from discarding
> > the device information from the fault path.
> > 
> 
> In case I didn't make it clear, what I talked about is just about having iommu
> core to report IOPF per domain handler vs. per device handler while this
> design choice doesn't change what the fault data should include (device,
> pasid, addr, etc.). i.e. it always includes all the information provided by the
> iommu driver no matter how the fault is reported upwards.

Right thanks, I misunderstood.

Thanks,
Jean

> 
> e.g. with iommufd it is iommufd to register a IOPF handler per managed
> domain and receive IOPF on those domains. If necessary, iommufd further
> forwards to userspace including device cookie according to the fault data.
> 
> Thanks
> Kevin
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