Re: [PATCH v4] arm64: errata: Fix exec handling in erratum 1418040 workaround

From: D Scott Phillips <scott@os.amperecomputing.com>
To: linux-arm-kernel@lists.infradead.org
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Marc Zyngier <maz@kernel.org>, Will Deacon <will@kernel.org>,
	Darren Hart <darren@os.amperecomputing.com>,
	patches@amperecomputing.com
Subject: Re: [PATCH v4] arm64: errata: Fix exec handling in erratum 1418040 workaround
Date: Fri, 17 Dec 2021 13:36:39 -0800	[thread overview]
Message-ID: <86lf0iud5k.fsf@scott-ph-mail.amperecomputing.com> (raw)
In-Reply-To: <20211217211920.2004032-1-scott@os.amperecomputing.com>

D Scott Phillips <scott@os.amperecomputing.com> writes:

> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index aacf2f5559a8..b37ff23e625e 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -439,34 +439,23 @@ static void entry_task_switch(struct task_struct *next)
>  
>  /*
>   * ARM erratum 1418040 handling, affecting the 32bit view of CNTVCT.
> - * Assuming the virtual counter is enabled at the beginning of times:
> - *
> - * - disable access when switching from a 64bit task to a 32bit task
> - * - enable access when switching from a 32bit task to a 64bit task
> + * Ensure access is disabled when switching to a 32bit task, ensure
> + * access is enabled when switching to a 64bit task.
>   */
> -static void erratum_1418040_thread_switch(struct task_struct *prev,
> -					  struct task_struct *next)
> +static void erratum_1418040_thread_switch(struct task_struct *next)
>  {
> -	bool prev32, next32;
> -	u64 val;
> -
> -	if (!IS_ENABLED(CONFIG_ARM64_ERRATUM_1418040))
> -		return;
> +	preempt_disable();
>  
> -	prev32 = is_compat_thread(task_thread_info(prev));
> -	next32 = is_compat_thread(task_thread_info(next));
> -
> -	if (prev32 == next32 || !this_cpu_has_cap(ARM64_WORKAROUND_1418040))
> +	if (!IS_ENABLED(CONFIG_ARM64_ERRATUM_1418040) ||
> +	    !this_cpu_has_cap(ARM64_WORKAROUND_1418040))
>  		return;

Oops, missing preempt_enable() here.

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