linux-kernel - Re: [PATCH v3] arm64: add the printing of tpidr_elx in __show

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Date:   Thu, 5 May 2022 11:51:48 +0100
From:   Will Deacon <will@...nel.org>
To:     Zhen Lei <thunder.leizhen@...wei.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] arm64: add the printing of tpidr_elx in __show_regs()

On Thu, May 05, 2022 at 05:56:40PM +0800, Zhen Lei wrote:
> Commit 7158627686f0 ("arm64: percpu: implement optimised pcpu access
> using tpidr_el1") and commit 6d99b68933fb ("arm64: alternatives: use
> tpidr_el2 on VHE hosts") use tpidr_elx to cache my_cpu_offset to optimize
> pcpu access. However, when performing reverse execution based on the
> registers and the memory contents in kdump, this information is sometimes
> required if there is a pcpu access.
> 
> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
> ---
>  arch/arm64/kernel/process.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> v2 --> v3:
> 1) Relace "switch (read_sysreg(CurrentEL))" statement with
>    "if (is_kernel_in_hyp_mode())" statement.
> 2) Change the register name to lowercase.
> 
> v1 --> v2:
> Directly print the tpidr_elx register of the current exception level.
> Avoid coupling with the implementation of 'my_cpu_offset'.
> 
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index 7fa97df55e3ad3f..7b6bccce9721c36 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -216,6 +216,11 @@ void __show_regs(struct pt_regs *regs)
>  	show_regs_print_info(KERN_DEFAULT);
>  	print_pstate(regs);
>  
> +	if (is_kernel_in_hyp_mode())
> +		printk("tpidr_el2 : %016llx\n", read_sysreg(tpidr_el2));
> +	else
> +		printk("tpidr_el1 : %016llx\n", read_sysreg(tpidr_el1));
> +

I'd still much prefer if we only printed this information for exceptions
taken in kernel mode. What use is this information for user faults?

We may only store an offset in the register today, but that could easily
change and nobody will come back and fix this code up.

Thanks,

Will