Re: [RFC PATCH] arch: ARM64: add isb before enable pan

[Will Deacon]

Hi,

On Fri, Oct 08, 2021 at 02:07:49PM +0800, Huangzhaoyang wrote:
> From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> 
> set_pstate_pan failure is observed in an ARM64 system occasionaly on a reboot
> test, which can be work around by a msleep on the sw context. We assume
> suspicious on disorder of previous instr of disabling SW_PAN and add an isb here.
> 
> PS:
> The bootup test failed with a invalid TTBR1_EL1 that equals 0x34000000, which is
> alike racing between on chip PAN and SW_PAN.

Sorry, but I'm struggling to understand the problem here. Please could you
explain it in more detail?

  - Why does a TTBR1_EL1 value of `0x34000000` indicate a race?
  - Can you explain the race that you think might be occurring?
  - Why does an ISB prevent the race?

> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> ---
>  arch/arm64/kernel/cpufeature.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index efed283..3c0de0d 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1663,6 +1663,7 @@ static void cpu_enable_pan(const struct arm64_cpu_capabilities *__unused)
>  	WARN_ON_ONCE(in_interrupt());
>  
>  	sysreg_clear_set(sctlr_el1, SCTLR_EL1_SPAN, 0);
> +	isb();
>  	set_pstate_pan(1);

SCTLR_EL1.SPAN only affects the PAN behaviour on taking an exception, which
is itself a context-synchronizing event, so I can't see why the ISB makes
any difference here (at least, for the purposes of PAN).

Thanks,

Will