On Tue, Apr 09, 2019 at 02:23:53PM +1000, Michael Ellerman wrote: > I'd much rather we use a trap with a specific immediate value. Otherwise > someone's going to waste time one day puzzling over why userspace is > doing mtmsr. It's data. We have other data in executable sections. Anyone who wonders about odd disassembly just hasn't realized they are disassembling data. > It would also complicate things if we ever wanted to emulate mtmsr. No, because it won't be executed. If I understand correctly, the only reason to choose an illegal, trap or privileged insn is to halt execution earlier rather than later when a program goes off in the weeds. > If we want something that is a trap rather than a nop then use 0x0fe50553. > > That's "compare the value in r5 with 0x553 and then trap unconditionally". > > It shows up in objdump as: > > 10000000: 53 05 e5 0f twui r5,1363 > > > The immediate can be anything, I chose that value to mimic the x86 value > Mathieu mentioned. > > There's no reason that instruction would ever be generated because the > immediate value serves no purpose. So it satisfies the "very unlikely > to appear" criteria AFAICS. Yes, looks fine to me, except that in VLE mode (do we care?) ".long 0x0fe50553" disassembles as 0: 0f e5 se_cmphl r5,r30 2: 05 53 se_mullw r3,r5 No illegal/trap/privileged insn there. ".long 0x0fe5000b" might be better to cover VLE. -- Alan Modra Australia Development Lab, IBM
