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

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Date:   Fri, 6 May 2022 11:09:46 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     "Leizhen (ThunderTown)" <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 Fri, May 06, 2022 at 04:16:55PM +0800, Leizhen (ThunderTown) wrote:
> 
> 
> On 2022/5/5 21:34, Leizhen (ThunderTown) wrote:
> > On 2022/5/5 21:26, Leizhen (ThunderTown) wrote:
> >> On 2022/5/5 21:04, Mark Rutland wrote:
> >>> 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());
> >>
> >> The function name is __show_regs(), so not using register name may not be good.
> >> In fact, some other architectures may also have this problem. If we use my_cpu_offset,
> >> we may need to put it in a public.
> > 
> > The other idea is to back up each my_cpu_offset in an array. In this way, the offset can
> > be queried through vmcore even if it is not printed.
> 
> Sorry, __per_cpu_offset[NR_CPUS] is always defined.

Surely that's in the vmcore already? It's just data in memory.

Thanks,
Mark.