[PATCH v2 -next] riscv: mm: remove redundant trampoline PGD for 64bit

[Nanyong Sun]

On 2021/8/14 6:08, Palmer Dabbelt wrote:
> On Mon, 02 Aug 2021 05:43:02 PDT (-0700), alex at ghiti.fr wrote:
>> Hi Nanyong,
>>
>> Le 28/07/2021 à 13:55, Alex Ghiti a écrit :
>>>
>>>
>>> Le 28/07/2021 à 04:49, Nanyong Sun a écrit :
>>>> Remove redundant trampoline PGD for 64bit and add more comment
>>>> for why 32bit systems need trampoline PGD.
>>>>
>>>>
>>>> +load_kernel_pgd:
>>>> +        /*
>>>> +         * Switch to kernel page tables.  A full fence is necessary
>>>> in order to
>>>> +         * avoid using the trampoline translations, which are only
>>>> correct for
>>>> +         * the first superpage.  Fetching the fence is guarnteed 
>>>> to work
>>>> +         * because that first superpage is translated the same way.
>>>> +         */
>>>> +        csrw CSR_SATP, a2
>>>> +        sfence.vma
>>>> +
>>>> +load_done:
>>>>       /* Set trap vector to spin forever to help debug */
>>>>       la a0, .Lsecondary_park
>>>>       csrw CSR_TVEC, a0
>>
>>
>> I suppose stvec was set this way to catch any problem with early_pg_dir,
>> you moved that and then this defeats this original purpose.
>
Hi Alex,

     I don't think so, before set early_pg_dir to satp, it's the 
physical address world, we must set stvec as

the first place in virtual address world we want jump to. And I don't 
think ".Lsecondary_park " can catch

problem of bad early_pg_dir, if the basic page table is wrong, CPU also 
can not go to the virtual address stored in stvec correctly.

More, in the original code, before set trampoline_pg_dir, what if the 
trampoline_pg_dir had a problem?

> Essentially.
>
> The specific issue is that the JTAG debug spec is defined (or at least 
> was when I was using it, it's been years since I've needed to do that) 
> in terms of committed instructions.  Thus if you end up in a position 
> where the processer is unable to commit an instruction you also lose 
> the ability to do anything meaningful with the debugger, thus 
> essentially locking up the system.
>
> The most common way to end up in a situation where the processor is 
> unable to commit an instruction is to have a fault with an invalid 
> trap vector: maybe dangling from M-mode, the last boot, reset, 
> whatever.  Then as soon as you take a trap the system locks up.  Any 
> trap before we have a working trap handler is a bug, but it's way 
> harder to debug things when the debugger doesn't function.
>
> There is of course no way to fundamentally prevent these sort of 
> no-commitable-instruction situations, but I got into the habbit of 
> just setting up a trivial trap entry point ASAP -- it probably took a 
> dozen rounds of trying to debug the debugger only to realize it was 
> per spec to hang, but that idiom eventually crept into pretty much 
> everything.
>
> Not sure if the debug spec is still written this way (or if debuggers 
> respect it), as I haven't had to use one in a while.
>
>>
>>
>>>>
>>>> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
>>>> index ac48742fa6fc..306fcb2334fa 100644
>>>> --- a/arch/riscv/mm/init.c
>>>> +++ b/arch/riscv/mm/init.c
>>>> @@ -219,13 +219,17 @@ unsigned long pfn_base __ro_after_init;
>>>>   EXPORT_SYMBOL(pfn_base);
>>>>   pgd_t swapper_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
>>>> +#ifndef CONFIG_64BIT
>>>>   pgd_t trampoline_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
>>>> +#endif /* CONFIG_64BIT */
>>
>>
>> As stated in Documentation/process/coding-style.rst, it is better to use
>> __maybe_unused rather than #ifdefs.
>>
>>
I'm afraid that __maybe_unused can not save one page memory here.
>>
>> Overall this version adds more complexity to assembly code than I
>> thought, but I don't see any way to improve that (which does not mean
>> there isn't!).
>>
>> Thanks,
>>
>> Alex
>>
Thanks for your review, let me figure out a better solution.
>>
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