On Fri, May 01, 2020 at 11:02:48AM +0200, Ard Biesheuvel wrote:

On Fri, 1 May 2020 at 10:54, Lorenzo Pieralisi
[off-list ref] wrote:

quoted

On Fri, May 01, 2020 at 10:30:11AM +0200, Ard Biesheuvel wrote:

quoted

On Fri, 17 Jan 2020 at 13:32, Lorenzo Pieralisi
[off-list ref] wrote:

quoted

On Fri, Jan 17, 2020 at 12:14:49PM +0000, Will Deacon wrote:

quoted

On Tue, Jan 14, 2020 at 08:14:11PM +0800, Hanjun Guo wrote:

quoted

The IORT specification [0] (Section 3, table 4, page 9) defines the
'Number of IDs' as 'The number of IDs in the range minus one'.

However, the IORT ID mapping function iort_id_map() treats the 'Number
of IDs' field as if it were the full IDs mapping count, with the
following check in place to detect out of boundary input IDs:

InputID >= Input base + Number of IDs

This check is flawed in that it considers the 'Number of IDs' field as
the full number of IDs mapping and disregards the 'minus one' from
the IDs count.

The correct check in iort_id_map() should be implemented as:

InputID > Input base + Number of IDs

this implements the specification correctly but unfortunately it breaks
existing firmwares that erroneously set the 'Number of IDs' as the full
IDs mapping count rather than IDs mapping count minus one.

e.g.

PCI hostbridge mapping entry 1:
Input base:  0x1000
ID Count:    0x100
Output base: 0x1000
Output reference: 0xC4  //ITS reference

PCI hostbridge mapping entry 2:
Input base:  0x1100
ID Count:    0x100
Output base: 0x2000
Output reference: 0xD4  //ITS reference

Two mapping entries which the second entry's Input base = the first
entry's Input base + ID count, so for InputID 0x1100 and with the
correct InputID check in place in iort_id_map() the kernel would map
the InputID to ITS 0xC4 not 0xD4 as it would be expected.

Therefore, to keep supporting existing flawed firmwares, introduce a
workaround that instructs the kernel to use the old InputID range check
logic in iort_id_map(), so that we can support both firmwares written
with the flawed 'Number of IDs' logic and the correct one as defined in
the specifications.

[...]

quoted

quoted

I just ran into this while playing with the LX2160 I received this week.

Side note: that firmware must be updatable or there is something I am
missing in relation to the ongoing ITS/SMMU mapping discussions.

Sure. But they are following the spec, and they use num_ids = 0x0 for
regions consisting of a single mapping. This is completely broken
without this patch.

Yes sure - I thought you were saying that the FW has issues with
the *current* kernel whereas you are asking if this fix can be
rewritten to remove the quirking mechanism - that's always a good
aim :)

quoted

quoted

I wonder if it would be better to detect the failure case dynamically,
rather than having these hardcoded quirks. It should be rather
straightforward to detect overlaps at the edges of these multi-range
mappings, in which case we could just let the spurious one (living at
the end of the region) be superseded by the correct one (living at the
start of the next region).

This could be done I think but probably requires some boot time parsing
to create some structure defining ranges (to avoid running the logic you
describe above every time a device has to be mapped).

It could be done much simpler than that: if iort_id_map() matches on
the last value of a region with size > 1 (so num_ids > 0), we return
the mapping but don't exit the loop. If we match it again, we use that
value, otherwise we use the tentative value from the first iteration.

We need to see how the code will look like in both cases, again, I am
not a priori against implementing it.

Instead of relying on ranges, we can precompute structures to define
for a device which mapping+mapping_index should be ignored, this
will save us from doing it dynamically (still require some additional
data).

quoted

Given that I have not heard of any regressions on the existing crop
of platforms and the one you mentioned has FW that is not set in stone
I think we can live with the quirk mechanism for the time being,
what do you think ?

The more quirks we keep, the harder it becomes to push back on new ones.

So if we can fix this cleanly, I'd much prefer it.

I agree with that, more so given that this is not impossible to rewrite
more cleanly.

Thanks,
Lorenzo

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