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Message-ID: <2bf90f7c-a026-488a-8a27-26f571b3d02a@arm.com>
Date: Fri, 29 Aug 2025 11:42:01 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: "Ritesh Harjani (IBM)" <ritesh.list@...il.com>,
linux-arm-kernel@...ts.infradead.org
Cc: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
Mark Brown <broonie@...nel.org>, Ryan Roberts <ryan.roberts@....com>,
Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC 3/3] arm64/ptdump: Add ARM64_PTDUMP_CONSOLE
On 29/08/25 9:12 AM, Ritesh Harjani (IBM) wrote:
> Anshuman Khandual <anshuman.khandual@....com> writes:
>
>> On 26/08/25 9:16 PM, Ritesh Harjani (IBM) wrote:
>>> Anshuman Khandual <anshuman.khandual@....com> writes:
>>>
>>>> Enable early kernel page table dump for debug purpose when required via new
>>>> config ARM64_PDUMP_CONSOLE. This calls existing ptdump_walk() early on just
>>>> after ptdump has been initialized with ptdump_init().
>>>
>>> I happen to stumble upon this while looking for something else related
>>> to ptdump and was curious to understand where this will be really
>>> useful?
>>>
>>> So instead of dumping it via cat /sys/kernel/debug/kernel_page_tables,
>>> this will dump at early boot during arch setup and before start_kernel().
>>
>> Right, primarily before vmalloc() space gets crowded. Also this provides
>> an opportunity to do a diff between early boot and after boot kernel page
>> table states.
>
> Just want to understand this better - the diff here will mainly show the
> new page table entries for the remaining layout which will get populated
> in start_kernel() right? The existing mappings created during
> setup_arch() won't get changed right?
Correct.
>
>>>
>>> I was curious, since this anyway gets enabled only in debug kernels.
>>> There we can always just boot with minimal busybox image which can jump
>>> to shell quickly and dump the kernel page tables, correct?
>>
>> Here the kernel page table dump could happen earlier than that as well.
>>>
>>> Also is ARM64_PTDUMP_CONSOLE config option added on purpose? A kernel cmdline
>>> like early_ptdump=yes|1|true could come much handy, right?
>>
>> Currently this is just for arm64 platform but could be enabled in general
>> for other platforms as well. Yes, early_ptdump=yes|1|true will be useful
>> as well. ARM64_PTDUMP_CONSOLE just build guards the additional code. But
>> if required cmdline option "early_ptdump=" could just provide the runtime
>> switch and then this could always be built enabled on CONFIG_PTDUMP.
>>>
>>> (Since I am fixing few issues on powerpc ptdump - hence was just curious
>>> to know whether this can come useful for me too or not :) )
>>
>> This feature could be extended in general to other platforms. Let me know
>> if you find this useful for powerpc.
>
> Sure - as of now I don't have a direct usecase. But let me try to do
> an early page table dump by taking some inspiration from your patch to
> see if this adds value for powerpc case or not.
Sure, please do let us know how that goes.
> > Thanks for sharing the info.
>
> -ritesh
>
>>>
>>> Thanks!
>>> -ritesh
>>>
>>>>
>>>> Suggested-by: Ryan Roberts <ryan.roberts@....com>
>>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>>>> ---
>>>> arch/arm64/Kconfig.debug | 12 ++++++++++++
>>>> arch/arm64/include/asm/ptdump.h | 7 +++++++
>>>> arch/arm64/kernel/setup.c | 1 +
>>>> arch/arm64/mm/ptdump.c | 7 +++++++
>>>> 4 files changed, 27 insertions(+)
>>>>
>>>> diff --git a/arch/arm64/Kconfig.debug b/arch/arm64/Kconfig.debug
>>>> index 265c4461031f..0f8af0dd0f4c 100644
>>>> --- a/arch/arm64/Kconfig.debug
>>>> +++ b/arch/arm64/Kconfig.debug
>>>> @@ -20,4 +20,16 @@ config ARM64_RELOC_TEST
>>>> depends on m
>>>> tristate "Relocation testing module"
>>>>
>>>> +config ARM64_PTDUMP_CONSOLE
>>>> + bool "Dump early kernel page table"
>>>> + depends on DEBUG_KERNEL
>>>> + depends on ARCH_HAS_PTDUMP
>>>> + select PTDUMP
>>>> + help
>>>> + Enable this option to dump early kernel page table entries during
>>>> + boot using the PTDUMP framework. This helps in examining kernel's
>>>> + page table mapping entries and their attributes etc.
>>>> +
>>>> + If in doubt, say N.
>>>> +
>>>> source "drivers/hwtracing/coresight/Kconfig"
>>>> diff --git a/arch/arm64/include/asm/ptdump.h b/arch/arm64/include/asm/ptdump.h
>>>> index 27e774134e7f..81dc53ca9643 100644
>>>> --- a/arch/arm64/include/asm/ptdump.h
>>>> +++ b/arch/arm64/include/asm/ptdump.h
>>>> @@ -74,8 +74,15 @@ void __init ptdump_debugfs_register(struct ptdump_info *info, const char *name);
>>>> static inline void ptdump_debugfs_register(struct ptdump_info *info,
>>>> const char *name) { }
>>>> #endif /* CONFIG_PTDUMP_DEBUGFS */
>>>> +
>>>> +#ifdef CONFIG_ARM64_PTDUMP_CONSOLE
>>>> +void __init arm64_kernel_pgtable_dump(void);
>>>> +#else
>>>> +static inline void __init arm64_kernel_pgtable_dump(void) { }
>>>> +#endif /* CONFIG_ARM64_PTDUMP_CONSOLE */
>>>> #else
>>>> static inline void __init ptdump_init(void) { }
>>>> +static inline void __init arm64_kernel_pgtable_dump(void) { }
>>>> static inline void note_page(struct ptdump_state *pt_st, unsigned long addr,
>>>> int level, pteval_t val) { }
>>>> static inline void note_page_pte(struct ptdump_state *st, unsigned long addr, pte_t pte) { }
>>>> diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
>>>> index 0a3812c8e177..86bf7607d304 100644
>>>> --- a/arch/arm64/kernel/setup.c
>>>> +++ b/arch/arm64/kernel/setup.c
>>>> @@ -361,6 +361,7 @@ void __init __no_sanitize_address setup_arch(char **cmdline_p)
>>>> init_bootcpu_ops();
>>>> smp_init_cpus();
>>>> smp_build_mpidr_hash();
>>>> + arm64_kernel_pgtable_dump();
>>>>
>>>> #ifdef CONFIG_ARM64_SW_TTBR0_PAN
>>>> /*
>>>> diff --git a/arch/arm64/mm/ptdump.c b/arch/arm64/mm/ptdump.c
>>>> index c78e6b496dea..f6d22462add6 100644
>>>> --- a/arch/arm64/mm/ptdump.c
>>>> +++ b/arch/arm64/mm/ptdump.c
>>>> @@ -407,6 +407,13 @@ void __init ptdump_init(void)
>>>> ptdump_initialize();
>>>> }
>>>>
>>>> +#ifdef CONFIG_ARM64_PTDUMP_CONSOLE
>>>> +void __init arm64_kernel_pgtable_dump(void)
>>>> +{
>>>> + ptdump_walk(CONSOLE, &kernel_ptdump_info);
>>>> +}
>>>> +#endif
>>>> +
>>>> static int __init ptdump_debugfs_init(void)
>>>> {
>>>> ptdump_debugfs_register(&kernel_ptdump_info, "kernel_page_tables");
>>>> --
>>>> 2.25.1
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