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From:   Alexandre Ghiti <alexghiti@rivosinc.com>
Date:   Thu, 8 Jun 2023 09:49:44 +0200
Message-ID: <CAHVXubggydGfNu3OzcxWXREJbB+G9dmr9sFD7vXhVxbG-N58Pg@mail.gmail.com>
Subject: Re: [PATCH] riscv: reserve DTB before possible memblock allocation
To:     Conor Dooley <conor@kernel.org>
Cc:     Woody Zhang <woodylab@foxmail.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
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On Wed, Jun 7, 2023 at 8:17=E2=80=AFPM Conor Dooley <conor@kernel.org> wrot=
e:
>
> +CC Alex, you should take a look at this patch.
>
> On Wed, Jun 07, 2023 at 09:35:19PM +0800, Woody Zhang wrote:
> > It's possible that early_init_fdt_scan_reserved_mem() allocates memory
> > from memblock for dynamic reserved memory in `/reserved-memory` node.
> > Any fixed reservation must be done before that to avoid potential
> > conflicts.
> >
> > Reserve the DTB in memblock just after early scanning it.
>
> The rationale makes sense to me, I am just wondering what compelling
> reason there is to move it away from the memblock_reserve()s for the
> initd and vmlinux? Moving it above early_init_fdt_scan_reserved_mem()
> should be the sufficient minimum & would keep things together.
>
> Cheers,
> Conor.

Thanks Conor.

So the patch looks good to me.

But I find this fragile:

- we do not check memblock_reserve() return value to make sure the
reservation really happened (and quickly looking at the code, I'm not
even sure it returns an error if the region was already allocated).
- we have to make sure no memblock allocation happens before setup_bootmem(=
).
- we also have to check that no fixed memblock_reserve() happens after.

The last 2 points may sound natural, but we'll have to take great care
when adding some code around here. I'm working on an "early boot
document" and I'll add something about that, but a runtime thing would
be way better IMO.

You can add:

Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>

Thanks,

Alex

>
> >
> > Signed-off-by: Woody Zhang <woodylab@foxmail.com>
> > ---
> >  arch/riscv/kernel/setup.c | 10 ++++++++++
> >  arch/riscv/mm/init.c      |  9 ---------
> >  2 files changed, 10 insertions(+), 9 deletions(-)
> >
> > diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> > index 36b026057503..c147fa8da929 100644
> > --- a/arch/riscv/kernel/setup.c
> > +++ b/arch/riscv/kernel/setup.c
> > @@ -16,6 +16,7 @@
> >  #include <linux/console.h>
> >  #include <linux/screen_info.h>
> >  #include <linux/of_fdt.h>
> > +#include <linux/libfdt.h>
> >  #include <linux/sched/task.h>
> >  #include <linux/smp.h>
> >  #include <linux/efi.h>
> > @@ -256,6 +257,15 @@ static void __init parse_dtb(void)
> >               pr_err("No DTB passed to the kernel\n");
> >       }
> >
> > +     /*
> > +      * If DTB is built in, no need to reserve its memblock.
> > +      * Otherwise, do reserve it but avoid using
> > +      * early_init_fdt_reserve_self() since __pa() does
> > +      * not work for DTB pointers that are fixmap addresses
> > +      */
> > +     if (!IS_ENABLED(CONFIG_BUILTIN_DTB))
> > +             memblock_reserve(dtb_early_pa, fdt_totalsize(dtb_early_va=
));
> > +
> >  #ifdef CONFIG_CMDLINE_FORCE
> >       strscpy(boot_command_line, CONFIG_CMDLINE, COMMAND_LINE_SIZE);
> >       pr_info("Forcing kernel command line to: %s\n", boot_command_line=
);
> > diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> > index c6bb966e4123..f8c9a79acd94 100644
> > --- a/arch/riscv/mm/init.c
> > +++ b/arch/riscv/mm/init.c
> > @@ -254,15 +254,6 @@ static void __init setup_bootmem(void)
> >        */
> >       early_init_fdt_scan_reserved_mem();
> >
> > -     /*
> > -      * If DTB is built in, no need to reserve its memblock.
> > -      * Otherwise, do reserve it but avoid using
> > -      * early_init_fdt_reserve_self() since __pa() does
> > -      * not work for DTB pointers that are fixmap addresses
> > -      */
> > -     if (!IS_ENABLED(CONFIG_BUILTIN_DTB))
> > -             memblock_reserve(dtb_early_pa, fdt_totalsize(dtb_early_va=
));
> > -
> >       dma_contiguous_reserve(dma32_phys_limit);
> >       if (IS_ENABLED(CONFIG_64BIT))
> >               hugetlb_cma_reserve(PUD_SHIFT - PAGE_SHIFT);
> > --
> > 2.39.2
> >