Re: [PATCH] arm64: errata: Fix handling of 1418040 with late CPU onlining

From: Marc Zyngier <maz@kernel.org>
To: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>,
	Suzuki Poulose <suzuki.poulose@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Stephen Boyd <swboyd@chromium.org>,
	kernel-team@android.com, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] arm64: errata: Fix handling of 1418040 with late CPU onlining
Date: Fri, 06 Nov 2020 12:15:34 +0000	[thread overview]
Message-ID: <0df3bbc4d9060dbba49ba120d9d88e88@kernel.org> (raw)
In-Reply-To: <20201106114952.10032-1-will@kernel.org>

On 2020-11-06 11:49, Will Deacon wrote:
> In a surprising turn of events, it transpires that CPU capabilities
> configured as ARM64_CPUCAP_WEAK_LOCAL_CPU_FEATURE are never set as the
> result of late-onlining. Therefore our handling of erratum 1418040 does
> not get activated if it is not required by any of the boot CPUs, even
> though we allow late-onlining of an affected CPU.
> 
> In order to get things working again, replace the cpus_have_const_cap()
> invocation with an explicit check for the current CPU using
> this_cpu_has_cap().
> 
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
> Cc: Stephen Boyd <swboyd@chromium.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Suzuki Poulose <suzuki.poulose@arm.com>
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
> 
> Found by code inspection and compile-tested only, so I would really
> appreciate a second look.
> 
>  arch/arm64/include/asm/cpufeature.h | 2 ++
>  arch/arm64/kernel/process.c         | 5 ++---
>  2 files changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h
> b/arch/arm64/include/asm/cpufeature.h
> index f7e7144af174..c59c16a6ea8b 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -268,6 +268,8 @@ extern struct arm64_ftr_reg arm64_ftr_reg_ctrel0;
>  /*
>   * CPU feature detected at boot time based on feature of one or more 
> CPUs.
>   * All possible conflicts for a late CPU are ignored.
> + * NOTE: this means that a late CPU with the feature will *not* cause 
> the
> + * capability to be advertised by cpus_have_*cap()!
>   */
>  #define ARM64_CPUCAP_WEAK_LOCAL_CPU_FEATURE		\
>  	(ARM64_CPUCAP_SCOPE_LOCAL_CPU		|	\
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index 4784011cecac..a47a40ec6ad9 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -522,14 +522,13 @@ static void erratum_1418040_thread_switch(struct
> task_struct *prev,
>  	bool prev32, next32;
>  	u64 val;
> 
> -	if (!(IS_ENABLED(CONFIG_ARM64_ERRATUM_1418040) &&
> -	      cpus_have_const_cap(ARM64_WORKAROUND_1418040)))
> +	if (!IS_ENABLED(CONFIG_ARM64_ERRATUM_1418040))
>  		return;
> 
>  	prev32 = is_compat_thread(task_thread_info(prev));
>  	next32 = is_compat_thread(task_thread_info(next));
> 
> -	if (prev32 == next32)
> +	if (prev32 == next32 || !this_cpu_has_cap(ARM64_WORKAROUND_1418040))
>  		return;

It is not going to be cheap to switch between 32 and 64bit.
Hopefully it doesn't happen too often... :-(.

Acked-by: Marc Zyngier <maz@kernel.org>

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

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