Date:   Fri, 1 Oct 2021 10:08:16 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Suzuki K Poulose <suzuki.poulose@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org, maz@...nel.org,
        catalin.marinas@....com, mark.rutland@....com, james.morse@....com,
        leo.yan@...aro.org, mike.leach@...aro.org,
        mathieu.poirier@...aro.org, will@...nel.org, lcherian@...vell.com,
        coresight@...ts.linaro.org
Subject: Re: [PATCH v2 11/17] arm64: errata: Add workaround for TSB flush
 failures



On 9/22/21 5:33 PM, Suzuki K Poulose wrote:
> Hi Anshuman
> 
> On 22/09/2021 08:39, Anshuman Khandual wrote:
>>
>>
>> On 9/21/21 7:11 PM, Suzuki K Poulose 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.
>>
>> s/everytime/every time
> 
> Ack
> 
>>
>>>
>>> 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@...nel.org>
>>> Cc: Catalin Marinas <catalin.marinas@....com>
>>> Cc: Mathieu Poirier <mathieu.poirier@...aro.org>
>>> Cc: Mike Leach <mike.leach@...aro.org>
>>> Cc: Mark Rutland <mark.rutland@....com>
>>> Cc: Anshuman Khandual <anshuman.khandual@....com>
>>> Cc: Marc Zyngier <maz@...nel.org>
>>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>>> ---
> 
> ...
> 
>>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>>> index eac4030322df..0764774e12bb 100644
>>> --- a/arch/arm64/Kconfig
>>> +++ b/arch/arm64/Kconfig
>>> @@ -705,6 +705,37 @@ config ARM64_ERRATUM_2139208
>>>           If unsure, say Y.
>>>   +config ARM64_WORKAROUND_TSB_FLUSH_FAILURE
>>> +    bool
>>> +
>>> +config ARM64_ERRATUM_2054223
>>> +    bool "Cortex-A710: 2054223: workaround TSB instruction failing to flush trace"
>>> +    default y
>>> +    help
>>> +      Enable workaround for ARM Cortex-A710 erratum 2054223
>>> +
>>> +      Affected cores may fail to flush the trace data on a TSB instruction, when
>>> +      the PE is in trace prohibited state. This will cause losing a few bytes
>>> +      of the trace cached.
>>> +
>>> +      Workaround is to issue two TSB consecutively on affected cores.
>>> +
>>> +      If unsure, say Y.
>>> +
>>> +config ARM64_ERRATUM_2067961
>>> +    bool "Neoverse-N2: 2067961: workaround TSB instruction failing to flush trace"
>>> +    default y
>>> +    help
>>> +      Enable workaround for ARM Neoverse-N2 erratum 2067961
>>> +
>>> +      Affected cores may fail to flush the trace data on a TSB instruction, when
>>> +      the PE is in trace prohibited state. This will cause losing a few bytes
>>> +      of the trace cached.
>>> +
>>> +      Workaround is to issue two TSB consecutively on affected cores.
>>
>> Like I had mentioned in the previous patch, these descriptions here could
>> be just factored out inside ARM64_WORKAROUND_TSB_FLUSH_FAILURE instead.
> 
> Please see my response there.
> 
>>
>>> +
>>> +      If unsure, say Y.
>>> +
>>>   config CAVIUM_ERRATUM_22375
>>>       bool "Cavium erratum 22375, 24313"
>>>       default y
>>> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
>>> index 451e11e5fd23..1c5a00598458 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 +46,20 @@
>>>   #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. The barriers    \
>>> +         * don't have to be strictly back to back, as long as the    \
>>> +         * CPU is in trace prohibited state.                \
>>> +         */                                \
>>> +        if (cpus_have_final_cap(ARM64_WORKAROUND_TSB_FLUSH_FAILURE))    \
>>> +            __tsb_csync();                        \
>>> +        __tsb_csync();                            \
>>> +    } while (0)
>>> +
>>>   /*
>>>    * Generate a mask for array_index__nospec() that is ~0UL when 0 <= idx < sz
>>>    * and 0 otherwise.
>>> diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
>>> index ccd757373f36..bdbeac75ead6 100644
>>> --- a/arch/arm64/kernel/cpu_errata.c
>>> +++ b/arch/arm64/kernel/cpu_errata.c
>>> @@ -352,6 +352,18 @@ static const struct midr_range trbe_overwrite_fill_mode_cpus[] = {
>>>   };
>>>   #endif    /* CONFIG_ARM64_WORKAROUND_TRBE_OVERWRITE_FILL_MODE */
>>>   +#ifdef CONFIG_ARM64_WORKAROUND_TSB_FLUSH_FAILURE
>>> +static const struct midr_range tsb_flush_fail_cpus[] = {
>>> +#ifdef CONFIG_ARM64_ERRATUM_2067961
>>> +    MIDR_ALL_VERSIONS(MIDR_NEOVERSE_N2),
>>> +#endif
>>> +#ifdef CONFIG_ARM64_ERRATUM_2054223
>>> +    MIDR_ALL_VERSIONS(MIDR_CORTEX_A710),
>>> +#endif
>>> +    {},
>>> +};
>>> +#endif    /* CONFIG_ARM64_WORKAROUND_TSB_FLUSH_FAILURE */
>>> +
>>>   const struct arm64_cpu_capabilities arm64_errata[] = {
>>>   #ifdef CONFIG_ARM64_WORKAROUND_CLEAN_CACHE
>>>       {
>>> @@ -558,6 +570,13 @@ const struct arm64_cpu_capabilities arm64_errata[] = {
>>>           .type = ARM64_CPUCAP_WEAK_LOCAL_CPU_FEATURE,
>>>           CAP_MIDR_RANGE_LIST(trbe_overwrite_fill_mode_cpus),
>>>       },
>>> +#endif
>>> +#ifdef CONFIG_ARM64_WORKAROUND_TSB_FLUSH_FAILRE
>>> +    {
>>> +        .desc = "ARM erratum 2067961 or 2054223",
>>> +        .capability = ARM64_WORKAROUND_TSB_FLUSH_FAILURE,
>>> +        ERRATA_MIDR_RANGE_LIST(tsb_flush_fail_cpus),
>>> +    },
>>>   #endif
>>>       {
>>>       }
>>> diff --git a/arch/arm64/tools/cpucaps b/arch/arm64/tools/cpucaps
>>> index 1ccb92165bd8..2102e15af43d 100644
>>> --- a/arch/arm64/tools/cpucaps
>>> +++ b/arch/arm64/tools/cpucaps
>>> @@ -54,6 +54,7 @@ WORKAROUND_1463225
>>>   WORKAROUND_1508412
>>>   WORKAROUND_1542419
>>>   WORKAROUND_TRBE_OVERWRITE_FILL_MODE
>>> +WORKAROUND_TSB_FLUSH_FAILURE
>>>   WORKAROUND_CAVIUM_23154
>>>   WORKAROUND_CAVIUM_27456
>>>   WORKAROUND_CAVIUM_30115
>>>
>>
>> This adds all the required bits of these erratas in a single patch,
>> where as the previous work around had split all the required pieces
>> into multiple patches. Could we instead follow the same standard in
>> both the places ?
> 
> We could do this for this particular erratum as the work around is
> within the arm64 kernel code, unlike the other ones - where the TRBE
> driver needs a change.
> 
> So, there is a kind of dependency for the other two, which we don't
> in this particular case.
> 
> i.e, TRBE driver needs a cpucap number to implement the work around ->
> The arm64 kernel must define one, which we cant advertise yet until
> we have a TRBE work around.
> 
> Thus, they follow a 3 step model.
> 
>  - Define CPUCAP erratum
>  - TRBE driver work around
>  - Finally advertise to the user.
> 
> I don't think this one needs that.

Okay, understood.

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