[PATCH 1/4] arm64: debug: unmask PSTATE.D earlier
Catalin Marinas
catalin.marinas at arm.com
Tue Jul 19 07:23:01 PDT 2016
On Tue, Jul 19, 2016 at 03:07:37PM +0100, 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 path 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 at vger.kernel.org>
> Acked-by: Mark Rutland <mark.rutland at arm.com>
> Reported-by: Catalin Marinas <catalin.marinas at arm.com>
> Tested-by: Marc Zyngier <marc.zyngier at arm.com>
> Signed-off-by: Will Deacon <will.deacon at arm.com>
For the record (and for the whole series):
Reviewed-by: Catalin Marinas <catalin.marinas at arm.com>
Tested-by: Catalin Marinas <catalin.marinas at arm.com>
I'll queue them for 4.8.
Thanks.
--
Catalin
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