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    SubjectRe: [PATCH] MIPS: BMIPS: Reserve exception base to prevent corruption
    On Mon, Mar 01, 2021 at 08:19:38PM -0800, Florian Fainelli wrote:
    > BMIPS is one of the few platforms that do change the exception base.
    > After commit 2dcb39645441 ("memblock: do not start bottom-up allocations
    > with kernel_end") we started seeing BMIPS boards fail to boot with the
    > built-in FDT being corrupted.
    >
    > Before the cited commit, early allocations would be in the [kernel_end,
    > RAM_END] range, but after commit they would be within [RAM_START +
    > PAGE_SIZE, RAM_END].
    >
    > The custom exception base handler that is installed by
    > bmips_ebase_setup() done for BMIPS5000 CPUs ends-up trampling on the
    > memory region allocated by unflatten_and_copy_device_tree() thus
    > corrupting the FDT used by the kernel.
    >
    > To fix this, we need to perform an early reservation of the custom
    > exception that is going to be installed and this needs to happen at
    > plat_mem_setup() time to ensure that unflatten_and_copy_device_tree()
    > finds a space that is suitable, away from reserved memory.
    >
    > Huge thanks to Serget for analysing and proposing a solution to this
    > issue.
    >
    > Fixes: Fixes: 2dcb39645441 ("memblock: do not start bottom-up allocations with kernel_end")

    > Debugged-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
    > Reported-by: Kamal Dasu <kdasu.kdev@gmail.com>

    I'd change the order of these two tags...

    > Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
    > ---

    > Thomas,
    >
    > This is intended as a stop-gap solution for 5.12-rc1 and to be picked up
    > by the stable team for 5.11. We should find a safer way to avoid these
    > problems for 5.13 maybe.

    Thomas, could you join the discussion? If we had a more clever
    solution to reserve the exceptions table for each possibly affected
    platform this patch could have been omitted.

    >
    > arch/mips/bmips/setup.c | 22 ++++++++++++++++++++++
    > arch/mips/include/asm/traps.h | 2 ++
    > 2 files changed, 24 insertions(+)
    >
    > diff --git a/arch/mips/bmips/setup.c b/arch/mips/bmips/setup.c
    > index 31bcfa4e08b9..0088bd45b892 100644
    > --- a/arch/mips/bmips/setup.c
    > +++ b/arch/mips/bmips/setup.c
    > @@ -149,6 +149,26 @@ void __init plat_time_init(void)
    > mips_hpt_frequency = freq;
    > }
    >
    > +static void __init bmips_ebase_reserve(void)
    > +{
    > + phys_addr_t base, size = VECTORSPACING * 64;
    > +
    > + switch (current_cpu_type()) {
    > + default:
    > + case CPU_BMIPS4350:
    > + return;
    > + case CPU_BMIPS3300:
    > + case CPU_BMIPS4380:
    > + base = 0x0400;
    > + break;
    > + case CPU_BMIPS5000:
    > + base = 0x1000;
    > + break;
    > + }
    > +
    > + memblock_reserve(base, size);
    > +}
    > +
    > void __init plat_mem_setup(void)
    > {
    > void *dtb;
    > @@ -169,6 +189,8 @@ void __init plat_mem_setup(void)
    >
    > __dt_setup_arch(dtb);
    >
    > + bmips_ebase_reserve();
    > +
    > for (q = bmips_quirk_list; q->quirk_fn; q++) {
    > if (of_flat_dt_is_compatible(of_get_flat_dt_root(),
    > q->compatible)) {
    > diff --git a/arch/mips/include/asm/traps.h b/arch/mips/include/asm/traps.h
    > index 6aa8f126a43d..0ba6bb7f9618 100644
    > --- a/arch/mips/include/asm/traps.h
    > +++ b/arch/mips/include/asm/traps.h
    > @@ -14,6 +14,8 @@
    > #define MIPS_BE_FIXUP 1 /* return to the fixup code */
    > #define MIPS_BE_FATAL 2 /* treat as an unrecoverable error */
    >

    > +#define VECTORSPACING 0x100 /* for EI/VI mode */

    What about the same macro declared in arch/mips/kernel/traps.c? I'd suggest
    to remove it from there and explicitly #include this header file into
    the arch/mips/bmips/setup.c file.

    -Sergey

    > +
    > extern void (*board_be_init)(void);
    > extern int (*board_be_handler)(struct pt_regs *regs, int is_fixup);
    >
    > --
    > 2.25.1
    >

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