Re: [PATCH] iommu: arm-smmu: Set SCTLR.HUPCF bit

[Jordan Crouse <jcrouse@codeaurora.org>]

On Mon, Nov 26, 2018 at 07:31:48PM +0000, Will Deacon wrote:
> Hi Rob,
> 
> On Tue, Nov 13, 2018 at 08:12:35AM -0500, Rob Clark wrote:
> > On Tue, Nov 13, 2018 at 1:32 AM Will Deacon <will.deacon@arm.com> wrote:
> > > On Fri, Nov 09, 2018 at 01:01:55PM -0500, Rob Clark wrote:
> > > > On Mon, Oct 29, 2018 at 3:09 PM Will Deacon <will.deacon@arm.com> wrote:
> > > > > On Thu, Sep 27, 2018 at 06:46:07PM -0400, Rob Clark wrote:
> > > > > > We seem to need to set either this or CFCFG (stall), otherwise gpu
> > > > > > faults trigger problems with other in-flight transactions from the
> > > > > > GPU causing CP errors, etc.
> > > > > >
> > > > > > In the ARM SMMU spec, the 'Hit under previous context fault' bit is
> > > > > > described as:
> > > > > >
> > > > > >  '0' - Stall or terminate subsequent transactions in the presence
> > > > > >        of an outstanding context fault
> > > > > >  '1' - Process all subsequent transactions independently of any
> > > > > >        outstanding context fault.
> > > > > >
> > > > > > Since we don't enable CFCFG (stall) the behavior of terminating
> > > > > > other transactions makes sense.  And is probably not what we want
> > > > > > (and definately not what we want for GPU).
> > > > > >
> > > > > > Signed-off-by: Rob Clark <robdclark@gmail.com>
> > > > > > ---
> > > > > > So I hit this issue a long time back on 820 (msm8996) and at the
> > > > > > time I solved it with a patch that enabled CFCFG.  And it resurfaced
> > > > > > more recently on sdm845.  But at the time CFCFG was rejected, iirc
> > > > > > because of concern that it would cause problems on other non-qcom
> > > > > > arm smmu implementations.  And I think I forgot to send this version
> > > > > > of the solution.
> > > > > >
> > > > > > If enabling HUPCF is anticipated to cause problems on other ARM
> > > > > > SMMU implementations, I think I can come up with a variant of this
> > > > > > patch which conditionally enables it for snapdragon.
> > > > > >
> > > > > > Either way, I'd really like to get some variant of this fix merged
> > > > > > (and probably it would be a good idea for stable kernel branches
> > > > > > too), since current behaviour with the GPU means faults turn into
> > > > > > a fantastic cascade of fail.
> > > > >
> > > > > Can you describe how this fantastic cascade of fail improves with this
> > > > > patch, please? If you're getting context faults then something has already
> > > > > gone horribly wrong, so I'm trying to work out how this improves things.
> > > > >
> > > >
> > > > There are plenty of cases where getting iommu faults with a GPU is
> > > > "normal", or at least not something the kernel or even GL driver can
> > > > control.
> > >
> > > Such as? All the mainline driver does is print a diagnostic and clear the
> > > fault, which doesn't seem generally useful.
> > 
> > it is useful to debug the fault ;-)
> > 
> > Although eventually we'll want to be able to do more than that, like
> > have the fault trigger bringing in pages of a mmap'd file and that
> > sort of thing.
> 
> Right, and feels very strange to me if we have this bit set because it
> means that your fault is no longer synchronous and therefore diverges
> from the fault model that you get from the CPU, where you certainly
> wouldn't expect stores appearing in the program after a faulting load to
> be visible in memory. However, thinking harder about it, I suppose we're
> already in a situation where the translations are handled out of order
> in the absence of barriers, so maybe it's not the end of the world.
> 
> Could you dump the FSR value that you see in the fault handler, please?
> From my reading of the architecture spec, as long as clear all of the
> fault bits in the irq handler, then your machine shouldn't die like it
> does with HUPCFG=CFCFG=0..
> 
> > > > With this patch, you still get the iommu fault, but it doesn't cause
> > > > the gpu to crash.  But without it, other memory accesses in flight
> > > > while the fault occurs, like the GPU command-processor reading further
> > > > ahead in the cmdstream to setup next draw, would return zero's,
> > > > causing the GPU to crash or get into a bad state.
> > >
> > > I get that part, but I don't understand why we're seeing faults in the first
> > > place and I worry that this patch is just the tip of the iceberg. It's also
> > > not clear that processing subsequent transactions is always the right thing
> > > to do in a world where we actually want to report (and handle) synchronous
> > > faults from devices.
> > 
> > Sure, it is a bug.. but it can be an application bug that is not
> > something the userspace GL driver or kernel could do anything about.
> > We shouldn't let this kill the GPU.  If the application didn't have
> > this much control, we wouldn't need an IOMMU in the first place[1].
> > With opencl compute, the userspace controlled shader has full blown
> > pointers to GPU memory.
> > 
> > And even in cases where it is a userspace GL driver bug, having some
> > robustness to not completely kill the GPU makes debugging much easier.
> > Something I do a lot when bringing up support for a new generation of
> > GPU.
> > 
> > I'm having a hard time understanding your objection to this.
> > Returning zero's for non-faulting transactions is a *really bad idea*.
> 
> Wait -- who said anything about returning zeroes? Where does that behaviour
> appear in the architecture?

I _think_ it is the bus implementation that returns zero on a terminated
transaction but the effect on the GPU is the same.

Jordan

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