Bug 689

Summary:	MMU PortInterface raises addr_ok_o even on a misaligned access exception
Product:	Libre-SOC's first SoC	Reporter:	Cesar Strauss <cestrauss>
Component:	Source Code	Assignee:	Luke Kenneth Casson Leighton <lkcl>
Status:	CONFIRMED ---
Severity:	major	CC:	libre-soc-bugs
Priority:	High
Version:	unspecified
Hardware:	PC
OS:	Linux
See Also:	https://bugs.libre-soc.org/show_bug.cgi?id=491 https://bugs.libre-soc.org/show_bug.cgi?id=465 https://bugs.libre-soc.org/show_bug.cgi?id=450
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Description Cesar Strauss 2021-09-12 14:20:23 BST

In pymem.py, it is written: "conversely: addr_ok_o must *ONLY* be asserted if there is a HUNDRED PERCENT guarantee that the memory request will be fulfilled."

When running test_issuer_mmu.py, on the LDSTExceptionTestCase(), I noticed that addr_ok_o is unexpectedly raised, on the same cycle as exc_happened (run "gtkwave issuer_simulator.gtkw", see "ld/st port interface" section).

I think this wasn't supposed to happen, even if the LDSTCompUnit didn't seem to actually care in this case.

Comment 1 Luke Kenneth Casson Leighton 2021-09-12 14:33:59 BST

yes, that's critically important that no possible indicator of success or
permitting of proceeding to a stage that REQUIRES 100% guaranteed completion.

this is down to the "House Contract Of Sale" protocol, which is:

* Offer
* Exchange
* Complete.

Wishbone is a "TAKE IT OR LEAVE IT" Contract.  you get someone walking
up to you on the street, saying, "buy my goods at reduced price! take it
or leave it!".

Speculative multi-issue execution *cannot* work on top of a take-it-or-leave-it
Contract.  you *have* to have a "conditional offer" phase. ONLY past the
"Exchange" point does it become a 100% cast-iron guaranteed requirement to
"Complete".

addr_ok_o is is part of the "Exchange" process, stating
that there are no objections to proceeding to "Complete"

if addr_ok_o is being raise at the *same time* as an indication that the
"Offer" is *REJECTED* (exc_happened) then that's extremely serious.

Comment 2 Cesar Strauss 2021-09-12 15:06:50 BST

Indeed.

It can also be more easily seen to happen with test_misalign_mmu() in src/soc/experiment/test/test_ldst_pi.py

I guess it has to do with LoadStore1 being derived from PortInterfaceBase, which seems to unconditionally raise addr_ok_o on receiving an addr.ok. Maybe it should somehow consult its derived class before doing so.

Comment 3 Luke Kenneth Casson Leighton 2021-09-12 16:48:56 BST

exceptions are the way that the "Offer" is refused. because we are only
just now starting to add exceptions, there will be a lot of areas that
need to support the "House Sale" protocol.

up until now, if a problem ever occurred, it was completely ignored!
there is even no error-checking on any wishbone bus exchanges, only
acknowledgements.