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

Leizhen (ThunderTown)
2022-05-05T13:28:09Z

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
>>
> .