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

[Sunnanyong (Nanyong Sun, Intelligent Computing Solution Development Dep)]

在 2021/6/29 21:01, Alex Ghiti 写道:

> Hi Nanyong,
>
> Le 27/05/2021 à 16:48, Nanyong Sun a écrit :
> > Remove redundant trampoline PGD for 64bit and add more comment
> > for why 32bit systems need trampoline PGD.
> >
> > There was a patch and discussion similar to this,refer to
> > the link [1][2].
> >
> > The trampoline PGD is redundant for 64bit systems because:
> > 1. The early PGD covers the entire kernel mapping. Directly
> > loading early PGD can achieve the result in boot stage.
> > A more trampoline PGD makes code hard to understand.
> > 2. Directly loading early PGD is safe in 64bit systems since
> > the kernel virtual address starts as 0xFFFFxxxxxxxxxxxx,
> > which has a very big gap with RAM address.It won't fall into
> > the corner case that 32bit system worrys.
> > 3. Remove redundant trampoline PGD can benefit to code maintaince,
> > because 64bit systems have more page table levels.For example:
> > If we want to support SV48 which has 4 page table levels, we have
> > to add a trampoline_pud and insert it before trampoline_pmd.
>
> I do agree with your last point about code maintenance and this would 
> be a welcome improvement before I respin my sv48 series.
>
> Some comments below though.
>
> > Reference link:
> > [1]https://lore.kernel.org/linux-riscv/20190325092234.5451-4-anup.patel@xxxxxxx/ 
> >
> > [2]https://lkml.org/lkml/2019/3/28/147
> >
> > Signed-off-by: Nanyong Sun <sunnanyong@xxxxxxxxxx>
> > ---
> >   arch/riscv/kernel/head.S | 13 +++++++++++--
> >   arch/riscv/mm/init.c     | 21 +++++++--------------
> >   2 files changed, 18 insertions(+), 16 deletions(-)
> >
> > diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> > index 89cc58ab52b4..1897b17c5fcc 100644
> > --- a/arch/riscv/kernel/head.S
> > +++ b/arch/riscv/kernel/head.S
> > @@ -93,12 +93,18 @@ relocate:
> >       add a2, a2, a1
> >       csrw CSR_TVEC, a2
>
> This is not needed in 64-bit then.
I'm afraid this is still needed for 64-bit, which can convert the 
physical address world to the virtual address world.

I also have made a experiment, it could not boot up if this is deleted.

> Thanks,
>
> Alex
> .

Hi Alex,

    Very thanks for your carefully review, I have updated to version 
two just now : )