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

[Marc Zyngier]

On Wed, 28 Jul 2021 14:52:17 +0100,
Suzuki K Poulose <suzuki.poulose@xxxxxxx> 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@xxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Mathieu Poirier <mathieu.poirier@xxxxxxxxxx>
> Cc: Mike Leach <mike.leach@xxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> Cc: Marc Zyngier <maz@xxxxxxxxxx>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> ---
>  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?

> +			__tsb_csync();						\
> +			__tsb_csync();						\
> +		} else {							\
> +			__tsb_csync();						\
> +		}								\

nit: You could keep one unconditional __tsb_csync().

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.