On Sun, 13 Jan 2019 23:33:56 +1100 Balbir Singh <bsingharora@xxxxxxxxx> wrote: > On Sat, Jan 12, 2019 at 02:45:41AM -0600, Segher Boessenkool wrote: > > On Sat, Jan 12, 2019 at 12:09:14PM +1100, Balbir Singh wrote: > > > Could you please define interesting frame on top a bit more? > > > Usually the topmost return address is in LR > > > > There is no reliable way (other than DWARF unwind info) to find out > > where the value of LR at function entry currently lives (if > > anywhere). It may or may not be still available in LR, it may or > > may not be saved to the return stack slot. It can also live in > > some GPR, or in some other stack slot. > > > > (The same is true for all other registers). > > > > The only thing the ABI guarantees you is that you can find all > > stack frames via the back chain. If you want more you can use some > > heuristics and do some heroics (like GDB does), but this is not > > fully reliable. Using DWARF unwind info is, but that requires big > > tables. > > > > Thanks, so are you suggesting that a reliable stack is not possible on > ppc64le? Even with the restricted scope of the kernel? The LR value location is _always_ hard to determine for the topmost frame. This is not a problem for voluntarily sleeping tasks, because the topmost function will always be well known. It is a problem for tasks preempted by an interrupt, or those handling an exception, that's why these need to report "unreliable". Note that this is a very general problem, across _all_ RISC-like architectures. It should thus be handled as generically as possible. Torsten
