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    Subject[PATCH 3.16 088/346] arm64: debug: unmask PSTATE.D earlier
    3.16.39-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Will Deacon <will.deacon@arm.com>

    commit 2ce39ad15182604beb6c8fa8bed5e46b59fd1082 upstream.

    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.

    Acked-by: Mark Rutland <mark.rutland@arm.com>
    Reported-by: Catalin Marinas <catalin.marinas@arm.com>
    Tested-by: Marc Zyngier <marc.zyngier@arm.com>
    Signed-off-by: Will Deacon <will.deacon@arm.com>
    Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
    Tested-by: Catalin Marinas <catalin.marinas@arm.com>
    Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    arch/arm64/kernel/debug-monitors.c | 1 -
    arch/arm64/kernel/smp.c | 1 -
    arch/arm64/mm/proc.S | 2 ++
    3 files changed, 2 insertions(+), 2 deletions(-)

    --- a/arch/arm64/kernel/debug-monitors.c
    +++ b/arch/arm64/kernel/debug-monitors.c
    @@ -159,7 +159,6 @@ static int debug_monitors_init(void)
    /* Clear the OS lock. */
    on_each_cpu(clear_os_lock, NULL, 1);
    isb();
    - local_dbg_enable();

    /* Register hotplug handler. */
    __register_cpu_notifier(&os_lock_nb);
    --- a/arch/arm64/kernel/smp.c
    +++ b/arch/arm64/kernel/smp.c
    @@ -174,7 +174,6 @@ asmlinkage void secondary_start_kernel(v
    set_cpu_online(cpu, true);
    complete(&cpu_running);

    - local_dbg_enable();
    local_irq_enable();
    local_async_enable();

    --- a/arch/arm64/mm/proc.S
    +++ b/arch/arm64/mm/proc.S
    @@ -189,6 +189,8 @@ ENTRY(__cpu_setup)
    msr cpacr_el1, x0 // Enable FP/ASIMD
    mov x0, #1 << 12 // Reset mdscr_el1 and disable
    msr mdscr_el1, x0 // access to the DCC from EL0
    + isb // Unmask debug exceptions now,
    + enable_dbg // since this is per-cpu
    reset_pmuserenr_el0 x0 // Disable PMU access from EL0
    /*
    * Memory region attributes for LPAE:
    \
     
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