Hi Maciej, > Hmm, this looks consistent with the TX79 manual: > > "6.2.1 Virtual Address Space > > The C790 only implements 32 bits of virtual address space. There is no > requirement for address sign extension and no checking will be done on the > upper 32 bits of the address." > > and then say in the JAL instruction description: > > "I: GPR[31] 63..0 <- zero_extend (PC + 8)" > > It does not matter for the user mode where bit #31 is 0 and therefore both > zero-extension and sign-extension produce the same result, so the typical > PIC code sequence used to determine its own location, i.e.: > > la $2, 0f > bltzal $0, 0f > 0: > subu $2, $31, $2 > > will work correctly, not causing UB with the SUBU instruction. > > However it does cause complications for the kernel in that the value of > $ra retrieved cannot be readily used for 32-bit calculations and has to be > treated with SLL by 0 first. You'll have to audit the arch/mips subtree > for any such $ra use for calculation; hopefully are there's none. > > I wonder why they broke it like this -- was it a silly deliberate choice > or merely an oversight (erratum) they chose to document rather than fix? > For a change they do implement MFC0 with sign-extension, so retrieving > e.g. CP0.EPC will see kernel addresses correctly sign-extended. After some further tests, it appears that for $ra, save/restore works with both SW/LW and SW/LWU. Hence, $ra bits 63:32 do not seem to matter at all (as intended), and its sign-extension failure can therefore be disregarded. This is somewhat non-obvious since $ra is the only register that fails to sign-extend after kernel initialisation (and consequently trigger a trap). > Anyway, as noted above that shouldn't cause a problem with user software > and I think that any corruption you can see comes from elsewhere. You'll > have to paper this $ra non-sign-extension issue over somehow to proceed > though. During early kernel initialisation at least one other register (besides $ra) appears to fail the sign-extension test, and the error cannot be ignored. I will now try figure this out. Fredrik
