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

["Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>]

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.

> 
> ... 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
>>
> .
> 

-- 
Regards,
  Zhen Lei

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