Re: [PATCH v3] arm64: add the printing of tpidr_elx in __show_regs()

From: "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	 <linux-arm-kernel@lists.infradead.org>,
	<linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v3] arm64: add the printing of tpidr_elx in __show_regs()
Date: Fri, 6 May 2022 18:21:37 +0800	[thread overview]
Message-ID: <cc8180ee-1c17-aab4-c883-4f8794edceb5@huawei.com> (raw)
In-Reply-To: <YnTz6oFEQGDBrIpi@FVFF77S0Q05N>

On 2022/5/6 18:09, Mark Rutland wrote:
> 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@huawei.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.

Yes. There is also a definition in drivers/base/arch_numa.c. I didn't search the drivers directory
at first. I thought there would be no definition when CONFIG_HAVE_SETUP_PER_CPU_AREA=y.

crash> p -x __per_cpu_offset
__per_cpu_offset = $1 =
 {0xffff8003f460d000, 0xffff8003f4621000, 0xffff8003f4635000, 0xffff8003f4649000

> 
> Thanks,
> Mark.
> .
> 

-- 
Regards,
  Zhen Lei

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