[sw,dv] Backdoor write from DV (sw_symbol_backdoor_overwrite) does not work for main SRAM

[andreaskurth]

When DV code calls sw_symbol_backdoor_overwrite on a symbol that is located in main SRAM (such as a non-const global variable declared in C code), the updated value of the variable might not be visible to SW running on Ibex.

One instance of this problem caused this spinwait in the rv_dm_access_after_wakeup test to not exit

opentitan/sw/device/tests/rv_dm_access_after_wakeup.c

Line 95 in 33e462e

OTTF_WAIT_FOR(kSoftwareBarrier == 1, kSoftwareBarrierTimeoutUsec);

When the variable is instead located in flash, which can be achieved by qualifying a global variable as const in C code, the value written by DV code is visible to SW code running on Ibex as expected. FPGA tests, on the other hand, need a variable that can be modified by SW and thus cannot be located in flash (which is write-protected by PMP), so a separate variable is needed (see comment below).

[andreaskurth]

Placing this in Backlog as it's not critical for any of the currently active milestones -- the workaround via flash should suffice for now.

[pamaury]

I should point that this is the main reason why many sival tests use a different set of variables for DV and silicon: the DV ones are in flash and the silicon one in RAM. Fixing this would therefore simplify the code.