On Sat, Dec 12, 2020 at 11:36:35AM +0000, Maciej W. Rozycki wrote: > Unlike TLS pointer access, specifically using $3 as rt, which has been > made a part of the Linux ABI, they're not supposed to be referred with > pre-R2 code. After all RDHWR was only introduced with R2. > > Indeed there's emulation code in the kernel for those encodings even with > pre-R2 hardware, but it is not guaranteed to give sensible results, and > given the circumstances I'm not sure what it really is for, e.g. what is > SYNCI_Step for with processors that do not implement SYNCI? The cycle > counter register may be absent too, so even emulated accesses will return > rubbish; gettimeofday(2) is the standard interface. > > So I think we can safely ignore them, just as we can any ULR access with > rt != $3. Unlike standardised TLS pointer accesses such instructions > won't appear in compiler-generated code and whoever uses them in handcoded > assembly or otherwise generated machine code will have to make sure they > make sense for their application (yes, there's rubbish code out there, > e.g. Firefox has a JIT that unconditionally produces MIPS R2 code even if > the piece of software has been compiled for an older ISA revision, and > having not verified that the CPU it runs on actually supports the R2 ISA, > but that's just the usual imperfection of the world; you just can't fix it > all). Many thanks, that resolves it! I'll write it up in the commit message. Fredrik
