On 3/23/21 5:24 AM, Mark Rutland wrote: > On Fri, Mar 19, 2021 at 05:03:09PM -0500, Madhavan T. Venkataraman wrote: >> I solved this by using existing functions logically instead of inventing a >> dummy function. I initialize pt_regs->stackframe[1] to an existing function >> so that the stack trace will not show a 0x0 entry as well as the kernel and >> gdb will show identical stack traces. >> >> For all task stack traces including the idle tasks, the stack trace will >> end at copy_thread() as copy_thread() is the function that initializes the >> pt_regs and the first stack frame for a task. > > I don't think this is a good idea, as it will mean that copy_thread() > will appear to be live in every thread, and therefore will not be > patchable. > > There are other things people need to be aware of when using an external > debugger (e.g. during EL0<->ELx transitions there are periods when X29 > and X30 contain the EL0 values, and cannot be used to unwind), so I > don't think there's a strong need to make this look prettier to an > external debugger. > OK. >> For EL0 exceptions, the stack trace will end with vectors() as vectors >> entries call the EL0 handlers. >> >> Here are sample stack traces (I only show the ending of each trace): >> >> Idle task on primary CPU >> ======================== >> >> ... >> [ 0.022557] start_kernel+0x5b8/0x5f4 >> [ 0.022570] __primary_switched+0xa8/0xb8 >> [ 0.022578] copy_thread+0x0/0x188 >> >> Idle task on secondary CPU >> ========================== >> >> ... >> [ 0.023397] secondary_start_kernel+0x188/0x1e0 >> [ 0.023406] __secondary_switched+0x40/0x88 >> [ 0.023415] copy_thread+0x0/0x188 >> >> All other kernel threads >> ======================== >> >> ... >> [ 13.501062] ret_from_fork+0x10/0x18 >> [ 13.507998] copy_thread+0x0/0x188 >> >> User threads (EL0 exception) >> ============ >> >> write(2) system call example: >> >> ... >> [ 521.686148] vfs_write+0xc8/0x2c0 >> [ 521.686156] ksys_write+0x74/0x108 >> [ 521.686161] __arm64_sys_write+0x24/0x30 >> [ 521.686166] el0_svc_common.constprop.0+0x70/0x1a8 >> [ 521.686175] do_el0_svc+0x2c/0x98 >> [ 521.686180] el0_svc+0x2c/0x70 >> [ 521.686188] el0_sync_handler+0xb0/0xb8 >> [ 521.686193] el0_sync+0x17c/0x180 >> [ 521.686198] vectors+0x0/0x7d8 > > [...] > >> If you approve, the above will become RFC Patch v3 1/8 in the next version. > > As above, I don't think we should repurpose an existing function here, > and my preference is to use 0x0. > OK. >> Let me know. >> >> Also, I could introduce an extra frame in the EL1 exception stack trace that >> includes vectors so the stack trace would look like this (timer interrupt example): >> >> call_timer_fn >> run_timer_softirq >> __do_softirq >> irq_exit >> __handle_domain_irq >> gic_handle_irq >> el1_irq >> vectors >> >> This way, if the unwinder finds vectors, it knows that it is an exception frame. > > I can see this might make it simpler to detect exception boundaries, but > I suspect that we need other information anyway, so this doesn't become > all that helpful. For EL0<->EL1 exception boundaries we want to > successfully terminate a robust stacktrace whereas for EL1<->EL1 > exception boundaries we want to fail a robust stacktrace. > > I reckon we have to figure that out from the el1_* and el0_* entry > points (which I am working to reduce/simplify as part of the entry > assembly conversion to C). With that we can terminate unwind at the > el0_* parts, and reject unwinding across any other bit of .entry.text. > OK. That is fine. Thanks. Madhavan
