Re: [PATCH 10/10] arm64: errata: Add workaround for TSB flush failures

[Suzuki K Poulose <suzuki.poulose@arm.com>]

On 29/07/2021 10:55, Marc Zyngier wrote:
> On Wed, 28 Jul 2021 14:52:17 +0100,
> Suzuki K Poulose <suzuki.poulose@arm.com> wrote:
>>
>> Arm Neoverse-N2 (#2067961) and Cortex-A710 (#2054223) suffers
>> from errata, where a TSB (trace synchronization barrier)
>> fails to flush the trace data completely, when executed from
>> a trace prohibited region. In Linux we always execute it
>> after we have moved the PE to trace prohibited region. So,
>> we can apply the workaround everytime a TSB is executed.
>>
>> The work around is to issue two TSB consecutively.
>>
>> NOTE: This errata is defined as LOCAL_CPU_ERRATUM, implying
>> that a late CPU could be blocked from booting if it is the
>> first CPU that requires the workaround. This is because we
>> do not allow setting a cpu_hwcaps after the SMP boot. The
>> other alternative is to use "this_cpu_has_cap()" instead
>> of the faster system wide check, which may be a bit of an
>> overhead, given we may have to do this in nvhe KVM host
>> before a guest entry.
>>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
>> Cc: Mike Leach <mike.leach@linaro.org>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
>> Cc: Marc Zyngier <maz@kernel.org>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
>> ---
>>   Documentation/arm64/silicon-errata.rst |  4 ++++
>>   arch/arm64/Kconfig                     | 31 ++++++++++++++++++++++++++
>>   arch/arm64/include/asm/barrier.h       | 17 +++++++++++++-
>>   arch/arm64/kernel/cpu_errata.c         | 19 ++++++++++++++++
>>   arch/arm64/tools/cpucaps               |  1 +
>>   5 files changed, 71 insertions(+), 1 deletion(-)
> 
> [...]
> 
>> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
>> index 451e11e5fd23..3bc1ed436e04 100644
>> --- a/arch/arm64/include/asm/barrier.h
>> +++ b/arch/arm64/include/asm/barrier.h
>> @@ -23,7 +23,7 @@
>>   #define dsb(opt)	asm volatile("dsb " #opt : : : "memory")
>>   
>>   #define psb_csync()	asm volatile("hint #17" : : : "memory")
>> -#define tsb_csync()	asm volatile("hint #18" : : : "memory")
>> +#define __tsb_csync()	asm volatile("hint #18" : : : "memory")
>>   #define csdb()		asm volatile("hint #20" : : : "memory")
>>   
>>   #ifdef CONFIG_ARM64_PSEUDO_NMI
>> @@ -46,6 +46,21 @@
>>   #define dma_rmb()	dmb(oshld)
>>   #define dma_wmb()	dmb(oshst)
>>   
>> +
>> +#define tsb_csync()								\
>> +	do {									\
>> +		/*								\
>> +		 * CPUs affected by Arm Erratum 2054223 or 2067961 needs	\
>> +		 * another TSB to ensure the trace is flushed.			\
>> +		 */								\
>> +		if (cpus_have_const_cap(ARM64_WORKAROUND_TSB_FLUSH_FAILURE)) {	\
> 
> Could this be made a final cap instead? Or do you expect this to be
> usable before caps have been finalised?

Good point. This can be final cap.

> 
>> +			__tsb_csync();						\
>> +			__tsb_csync();						\
>> +		} else {							\
>> +			__tsb_csync();						\
>> +		}								\
> 
> nit: You could keep one unconditional __tsb_csync().

I thought about that, I was worried if the CPU expects them back to back
without any other instructions in between them. Thinking about it a bit
more, it doesn't look like that is the case. I will confirm this and
change it accordingly.

Thanks
Suzuki

> 
> Thanks,
> 
> 	M.
>