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

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Date:   Thu, 5 May 2022 14:04:00 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Zhen Lei <thunder.leizhen@...wei.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        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));

If we care about the offset specifically, this would be simpler as:

	printk("cpu offset : 0x%016lx\n", __my_cpu_offset());

... which should do the right thing even if we repurpose the TPIDRs and move the offset elsewhere.

As Will says, we should only log this for !user_mode(regs), so it could
be placed in the block below, immediately before we print the kernel PC, i.e.

	if (!user_mode_regs) {
		printk("cpu offset : %016lx\n", __my_cpu_offset());
		printk("pc : %pS\n", (void *)regs->pc);
		printk("lr : %pS\n", (void *)ptrauth_strip_insn_pac(lr));
		...
	}

... or in a separate block which checks the same condition.

Thanks,
Mark.

> +
>  	if (!user_mode(regs)) {
>  		printk("pc : %pS\n", (void *)regs->pc);
>  		printk("lr : %pS\n", (void *)ptrauth_strip_insn_pac(lr));
> -- 
> 2.25.1
>