Re: [PATCH V3] MIPS: Loongson64: Add kexec/kdump support

[Jiaxun Yang <jiaxun.yang@flygoat.com>]

在 2021/1/8 下午6:07, Jinyang He 写道:
> Hi, Thomas,
>
> On 01/08/2021 01:26 AM, Thomas Bogendoerfer wrote:
>>>>> --- a/arch/mips/kernel/relocate_kernel.S
>>>>> +++ b/arch/mips/kernel/relocate_kernel.S
>>>>> @@ -6,6 +6,7 @@
>>>>>
>>>>>    #include <asm/asm.h>
>>>>>    #include <asm/asmmacro.h>
>>>>> +#include <asm/cpu.h>
>>>>>    #include <asm/regdef.h>
>>>>>    #include <asm/mipsregs.h>
>>>>>    #include <asm/stackframe.h>
>>>>> @@ -133,6 +134,33 @@ LEAF(kexec_smp_wait)
>>>>>    #else
>>>>>        sync
>>>>>    #endif
>>>>> +
>>>>> +#ifdef CONFIG_CPU_LOONGSON64
>> Is there a reason why you can't use the already existing infrastructure
>> the way cavium-octeon is doing it ? If you can't please explain why
>> so we can find a way to extend it. But having some sort of poking
>> loongson registers in generic MIPS code is a non starter.
>>
>> Thomas.
>>
>
> Unlike the cavium-octeon platform, the Loongson64 platform needs some 
> changes. Before the kernel starts, (before entering the kernel_entry), 
> each CPU has its own state (the SMP system). For Loongson64, only the 
> boot CPU will enter the kernel_entry, and other CPUs will query their 
> mailbox value in a loop. This is what the BIOS does for the CPU. Here 
> is different from cavium-octeon. All CPUs will enter the kernel_entry 
> on cavium-octeon platform. Then the kernel_entry_setup, the co-CPUs 
> will enter the query loop. I saw the kernel_entry_setup of other 
> platforms, such as ip27, malta, and generic. They are not like 
> cavium-octeon and the co-CPUs entering the loop may be earlier than 
> entering kernel_entry. So I have reason to guess that most SMP system 
> platform CPUs are similar to Loongson64.

Hi Jingyang,

As I commented before you may reuse play_dead logic in Loongson's smp.c.

>
> relocate_kernel.S is like BIOS doing s omething for the CPU. It allows 
> the boot CPU to start from the new kernel_entry and makes the co-CPUs 
> enter a loop. The already existing infrastructure may be more suitable 
> for non-smp platforms. Although we can do something with 
> plat_smp_ops.kexec_nonboot_cpu, more new problems will arise in that 
> case. The kexec process actually runs on a copy of relocate_kernel.S, 
> which will bring a lot of problems...

It won't be a problem as you can keep all data on-stack without external 
reference.

Thanks.

- Jiaxun

>
> Above all just my personal thoughts.
>
> Thanks,
> Jinyang
>