Hi Pratyush, On 31/07/17 11:40, Pratyush Anand wrote: > samples/hw_breakpoint/data_breakpoint.c passes with x86_64 but fails with > ARM64. Even though it has been NAKed previously on upstream [1, 2], I have > tried to come up with patches which can resolve it for ARM64 as well. > > I noticed that even perf step exception can go into an infinite loop if CPU > receives an interrupt while executing breakpoint/watchpoint handler. So, > event though we are not concerned about above test, we will have to find a > solution for the perf issue.
This caught my eye as I've been reworking the order the DAIF flags get set/cleared[0]. What causes your infinite loop? Is it single-stepping kernel_exit? If so patch 4 "arm64: entry.S mask all exceptions during kernel_exit" [1] may help. If its more like "single stepping something we didn't expect" you will get the same problem if we take an SError. (which with that series is unmasked ~all the time). Either way this looks like a new and exciting way of hitting the 'known issue' described in patch 12 [3]. Would disabling MDSCR_EL1.SS if we took an exception solve your problem? If so, I think we should add a new flag, 'TIF_KSINGLESTEP', causing us to save/restore MDSCR_EL1.SS into pt_regs on el1 exceptions. This would let us single-step without modifying the DAIF flags for the location we are stepping, and allow taking any kind of exception from that location. We should disable nested users of single-step, we can do that by testing the flag, print a warning then pretend we missed the breakpoint. (hence it needs to be separate from the user single-step flag). Thanks, James [0] https://www.spinics.net/lists/arm-kernel/msg596684.html [1] https://www.spinics.net/lists/arm-kernel/msg596686.html [2] https://www.spinics.net/lists/arm-kernel/msg596689.html
