RE: Not enough CXL HDM decoders in pass through host bridges (sort of)

[Dan Williams <dan.j.williams@intel.com>]

Jonathan Cameron wrote:
> Hi Dan,
> 
> I've finally been adding support for multiple HDM decoders in QEMU
> (need to implement the address decode at EPs, but have it working at
> HB and switch USP)
> 
> Whilst testing ran into a corner case on the kernel side of things.
> 
>            Host Bridge
>                 |
>             Root Port
>                 |
>            Switch USP
>                 |   
>         ________|_________       
>        |                  |
>      DSP0               DSP1
>        |                  |
>      Type3              Type3
> 
> 
> Previously I'd been testing this with either an interleave across the two
> Type3 devices or with just one in use (as I only had one HDM decoder in the SW USP
> so couldn't handle anything else)
> 
> Now I have lots of decoders, I added a simple test with two regions. One on each of
> the type 3 devices.
> 
> It fails on the second region (with a "no decoder available error") because...
> 
> It's trying to find an HDM decoder in the host bridge and the fake one used
> for a pass through decoder is 'already in use' by the first region.
> 
> I'm not sure we can simply skip the check in this case because cxld->region
> can only point at one region at a time and I haven't though through
> the impacts of that for a pass through decoder.  The cynic in me says
> that if this HB had more RPs, we'd have a maximum of 32 decoders, so
> just fake 32 of them instead of 1 and not worry about it any more but
> that feels like a hack and probably has side effects.
> 
> I thought I'd raise the issue first and think about a solution afterwards
> (and secretly hope it is fixed before I get to it ;)).

Hmm, in the case of no decoders in a host-bridge the root-decoders are
effectively mirrored at that level. I.e. root-decoders already support
the property of hosting multiple regions in a decoder so perhaps
tunneling them in this case would be a better model then establishing a
unique "passthrough decoder".

> Obviously I can avoid the whole thing by adding an RP and hence have
> actually decoders to use up on the host bridge which does fine for
> testing my QEMU work, but we still need to fix this up - unless I'm missing
> some subtlety.

I just find it difficult to believe that someone will build decoder-less
host bridges. The moment that you have multiple CFMWS windows to account
for ram vs pmem, type-2 vs type-3, etc... then it mandates multiple
distinct decoders at each level.

At a minimum I think this problem can be solved at leisurely place
unless someone can point to a non-QEMU example of such a platform to
raise the urgency.

> There is another question of whether we should make some effort to conserve
> decoders - so if we can just expand an existing one to cover a wider range?
> We can't do that after commit, but maybe there is a dance we could do to
> soft commit a bunch of regions, then hard commit them as a set.
> HDM decoders may be a precious resource on some systems even though CXL 3.0
> let's host bridges have 32 of them.  One to tackle only when it's a real problem
> though.

How would that work a practice? A switch can only target multiple
downstreams for a given address range if they are interleaved, and if
they are interleaved it's a single region.