linux-kernel - Re: [PATCH v4] riscv: Use PUD/P4D/PGD pages for the linear mapping

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Date:   Mon, 30 Jan 2023 07:48:04 -0600
From:   Rob Herring <robh+dt@...nel.org>
To:     Alexandre Ghiti <alexghiti@...osinc.com>
Cc:     Andrew Jones <ajones@...tanamicro.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>, Guo Ren <guoren@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH v4] riscv: Use PUD/P4D/PGD pages for the linear mapping

On Wed, Jan 25, 2023 at 6:13 AM Alexandre Ghiti <alexghiti@...osinc.com> wrote:
>
> On Wed, Jan 25, 2023 at 11:41 AM Andrew Jones <ajones@...tanamicro.com> wrote:
> >
> > On Mon, Jan 23, 2023 at 03:25:54PM +0100, Andrew Jones wrote:
> > > On Mon, Jan 23, 2023 at 12:28:02PM +0100, Alexandre Ghiti wrote:
> > > > During the early page table creation, we used to set the mapping for
> > > > PAGE_OFFSET to the kernel load address: but the kernel load address is
> > > > always offseted by PMD_SIZE which makes it impossible to use PUD/P4D/PGD
> > > > pages as this physical address is not aligned on PUD/P4D/PGD size (whereas
> > > > PAGE_OFFSET is).

[...]

> > > > diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
> > > > index f08b25195ae7..58107bd56f8f 100644
> > > > --- a/drivers/of/fdt.c
> > > > +++ b/drivers/of/fdt.c
> > > > @@ -891,12 +891,13 @@ const void * __init of_flat_dt_match_machine(const void *default_match,
> > > >  static void __early_init_dt_declare_initrd(unsigned long start,
> > > >                                        unsigned long end)
> > > >  {
> > > > -   /* ARM64 would cause a BUG to occur here when CONFIG_DEBUG_VM is
> > > > -    * enabled since __va() is called too early. ARM64 does make use
> > > > -    * of phys_initrd_start/phys_initrd_size so we can skip this
> > > > -    * conversion.
> > > > +   /*
> > > > +    * __va() is not yet available this early on some platforms. In that
> > > > +    * case, the platform uses phys_initrd_start/phys_initrd_size instead
> > > > +    * and does the VA conversion itself.
> > > >      */
> > > > -   if (!IS_ENABLED(CONFIG_ARM64)) {
> > > > +   if (!IS_ENABLED(CONFIG_ARM64) &&
> > > > +       !(IS_ENABLED(CONFIG_RISCV) && IS_ENABLED(CONFIG_64BIT))) {
> > >
> > > There are now two architectures, so maybe it's time for a new config
> > > symbol which would be selected by arm64 and riscv64 and then used here,
> > > e.g.
> > >
> > >   if (!IS_ENABLED(CONFIG_NO_EARLY_LINEAR_MAP)) {
> >
> > I see v5 left this as it was. Any comment on this suggestion?
>
> Introducing a config for this only use case sounds excessive to me,
> but I'll let Rob decide what he wants to see here.

Agreed. Can we just keep it as is here.

> > > >             initrd_start = (unsigned long)__va(start);
> > > >             initrd_end = (unsigned long)__va(end);

I think long term, we should just get rid of needing to do this part
in the DT code and let the initrd code do this.

Rob