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

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Message-ID: <CAL_JsqJdBGAOELLTQbfi6Q3r-AnS5Wf8VNf94BP6Y9TNvPHS-w@mail.gmail.com>
Date:   Mon, 30 Jan 2023 08:57:58 -0600
From:   Rob Herring <robh+dt@...nel.org>
To:     Andrew Jones <ajones@...tanamicro.com>
Cc:     Alexandre Ghiti <alexghiti@...osinc.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 Mon, Jan 30, 2023 at 8:19 AM Andrew Jones <ajones@...tanamicro.com> wrote:
>
> On Mon, Jan 30, 2023 at 07:48:04AM -0600, Rob Herring wrote:
> > 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.
>
> initrd code provides reserve_initrd_mem() for this and riscv calls
> it later on. afaict, this early setting in OF code is a convenience
> which architectures could be taught not to depend on, and then it
> could be removed. But, until then, some architectures will need to
> avoid it. As I commented downthread, I also don't want to go with
> a config anymore, but it'd be nice to keep arch-specifics out of
> here, so I've posted a patch changing __early_init_dt_declare_initrd
> to be a weak function.

If this was *always* going to be architecture specific, then I'd
agree. But I think there are better paths to refactor this. I don't
want to go back to a weak function and encourage more implementations
of __early_init_dt_declare_initrd().

Rob