On 19/07/16 12:19, Will Deacon wrote: > Clearing PSTATE.D is one of the requirements for generating a debug > exception. The arm64 booting protocol requires that PSTATE.D is set, > since many of the debug registers (for example, the hw_breakpoint > registers) are UNKNOWN out of reset and could potentially generate > spurious, fatal debug exceptions in early boot code if PSTATE.D was > clear. Once the debug registers have been safely initialised, PSTATE.D > is cleared, however this is currently broken for two reasons: > > (1) The boot CPU clears PSTATE.D in a postcore_initcall and secondary > CPUs clear PSTATE.D in secondary_start_kernel. Since the initcall > runs after SMP (and the scheduler) have been initialised, there is > no guarantee that it is actually running on the boot CPU. In this > case, the boot CPU is left with PSTATE.D set and is not capable of > generating debug exceptions. > > (2) In a preemptible kernel, we may explicitly schedule on the IRQ > return path to EL1. If an IRQ occurs with PSTATE.D set in the idle > thread, then we may schedule the kthread_init thread, run the > postcore_initcall to clear PSTATE.D and then context switch back > to the idle thread before returning from the IRQ. The exception > return patch will then restore PSTATE.D from the stack, and set it > again. > > This patch fixes the problem by moving the clearing of PSTATE.D earlier > to proc.S. This has the desirable effect of clearing it in one place for > all CPUs, long before we have to worry about the scheduler or any > exception handling. We ensure that the previous reset of MDSCR_EL1 has > completed before unmasking the exception, so that any spurious > exceptions resulting from UNKNOWN debug registers are not generated. > > Without this patch applied, the kprobes selftests have been seen to fail > under KVM, where we end up attempting to step the OOL instruction buffer > with PSTATE.D set and therefore fail to complete the step. > > Cc: <stable@xxxxxxxxxxxxxxx> > Reported-by: Catalin Marinas <catalin.marinas@xxxxxxx> > Signed-off-by: Will Deacon <will.deacon@xxxxxxx> Gave it a spin on both Juno and Seattle, both host and KVM guest, and everything went smoothly, so: Tested-by: Marc Zyngier <marc.zyngier@xxxxxxx> Thanks, M. -- Jazz is not dead. It just smells funny... -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html