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

[Alex Ghiti]

Hi Nanyong,

Le 8/09/2021 à 08:42, Nanyong Sun a écrit :
> On 2021/8/14 6:08, Palmer Dabbelt wrote:
> > On Mon, 02 Aug 2021 05:43:02 PDT (-0700), alex@xxxxxxxx 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.

But I think then that it loops forever at the stvec address which allows 
to know where the boot failed.

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

You're right but this debug 'feature' was not installed, I guess 
somebody had a hard time at some point with the early page table and not 
the trampoline :)

Anyway, I was just pointing that you 'broke' the current way things work 
and unless this is for an explicit good reason, that should not happen.

> > 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.

What do you mean?

> > > 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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