----- On Apr 9, 2019, at 5:29 AM, Alan Modra amodra@xxxxxxxxx wrote: > 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. Can you share with us the objdump output of ".long 0x0fe5000b" in VLE mode ? VLE mode support does not appear to be available in typical toolchains. Also, is VLE mode only for powerpc 32 be, or also for powerpc 64 be/le ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
