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Subject: Re: [PATCH 1/3] arm64: debug: unmask PSTATE.D earlier
To: Will Deacon <will.deacon@arm.com>, linux-eng@arm.com
References: <1468927195-24468-1-git-send-email-will.deacon@arm.com>
Cc: catalin.marinas@arm.com, stable@vger.kernel.org
From: Marc Zyngier <marc.zyngier@arm.com>
Message-ID: <578E245C.9010202@arm.com>
Date: Tue, 19 Jul 2016 14:00:12 +0100
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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@vger.kernel.org>
> Reported-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Will Deacon <will.deacon@arm.com>

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@arm.com>

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...