Subject: Re: [PATCH 1/4] powerpc/mmu: do MMU type discovery before crashkernel reservation

From: Ritesh Harjani (IBM) <[email-masked]>
Date: Fri, 31 Oct 2025 10:23:55 +0530

Sourabh Jain <[email-masked]> writes:

> Crashkernel reservation on high memory depends on the MMU type, so
> finalize the MMU type before calling arch_reserve_crashkernel().
>
> With the changes introduced here, early_radix_enabled() becomes usable
> and will be used in arch_reserve_crashkernel() in the upcoming patch.
>
> early_radix_enabled() depends on cur_cpu_spec->mmu_features to find
> out if the radix MMU is enabled. The radix MMU bit in mmu_features is
> discovered from the FDT and kernel configs. To make sure the MMU type is
> finalized before arch_reserve_crashkernel() is called, the function that
> scans the FDT and sets mmu_features, along with some bits from
> mmu_early_type_finalize(), has been moved above
> arch_reserve_crashkernel().
>

That is correct. mmu_features may as well get set from this path too...

early_init_dt_scan_cpus() -> 
   if (!dt_cpu_ftrs_in_use())
       -> check_cpu_features(node, "ibm_pa_features",...
                      cur_cpu_spec->mmu_features |= fp->mmu_features

...which I guess is controlled using CONFIG_PPC_DT_CPU_FTRS.

so it make sense to move those dependent paths above.

Overall the patch looks good to me. Added few minor nits below.

> Cc: Baoquan he <[email-masked]>
> Cc: Hari Bathini <[email-masked]>
> Cc: Madhavan Srinivasan <[email-masked]>
> Cc: Mahesh Salgaonkar <[email-masked]>
> Cc: Michael Ellerman <[email-masked]>
> Cc: Ritesh Harjani (IBM) <[email-masked]>
> Cc: Shivang Upadhyay <[email-masked]>
> Signed-off-by: Sourabh Jain <[email-masked]>
> ---
>  arch/powerpc/include/asm/book3s/64/mmu.h |  1 +
>  arch/powerpc/include/asm/mmu.h           |  1 +
>  arch/powerpc/kernel/prom.c               | 28 +++++++++++++-----------
>  arch/powerpc/mm/init_64.c                | 27 ++++++++++++++---------
>  4 files changed, 34 insertions(+), 23 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/book3s/64/mmu.h b/arch/powerpc/include/asm/book3s/64/mmu.h
> index 48631365b48c..7a3b2ff02041 100644
> --- a/arch/powerpc/include/asm/book3s/64/mmu.h
> +++ b/arch/powerpc/include/asm/book3s/64/mmu.h
> @@ -208,6 +208,7 @@ extern int mmu_vmemmap_psize;
>  
>  /* MMU initialization */
>  void mmu_early_init_devtree(void);
> +void mmu_early_type_finalize(void);

Minor nit:
Can we please rename this function to - mmu_early_init_vec5()?
Your naming isn't wrong, but it's just known that after vec5 call, we
finalize the mmu early init type.. So keeping this function name as
"mmu_early_init_vec5()" makes slightly more sense to me. 

And then the order of function declarations can also be kept like below - 

 /* MMU initialization */
 +void mmu_early_init_vec5(void);
 void mmu_early_init_devtree(void);

diff --git a/arch/powerpc/include/asm/mmu.h b/arch/powerpc/include/asm/mmu.h
 +static inline void mmu_early_init_vec5(void) { }
 static inline void mmu_early_init_devtree(void) { }

>  void hash__early_init_devtree(void);
>  void radix__early_init_devtree(void);
>  #ifdef CONFIG_PPC_PKEY
> diff --git a/arch/powerpc/include/asm/mmu.h b/arch/powerpc/include/asm/mmu.h
> index 5f9c5d436e17..c40dc6349e55 100644
> --- a/arch/powerpc/include/asm/mmu.h
> +++ b/arch/powerpc/include/asm/mmu.h
> @@ -384,6 +384,7 @@ extern void early_init_mmu_secondary(void);
>  extern void setup_initial_memory_limit(phys_addr_t first_memblock_base,
>  				       phys_addr_t first_memblock_size);
>  static inline void mmu_early_init_devtree(void) { }
> +static inline void mmu_early_type_finalize(void) { }
>  
>  static inline void pkey_early_init_devtree(void) {}
>  
> diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c
> index 9ed9dde7d231..db1615f26075 100644
> --- a/arch/powerpc/kernel/prom.c
> +++ b/arch/powerpc/kernel/prom.c
> @@ -853,6 +853,21 @@ void __init early_init_devtree(void *params)
>  	if (PHYSICAL_START > MEMORY_START)
>  		memblock_reserve(MEMORY_START, int_vector_size);
>  	reserve_kdump_trampoline();
> +
> +	DBG("Scanning CPUs ...\n");
> +
> +	dt_cpu_ftrs_scan();
> +
> +	/* Retrieve CPU related informations from the flat tree
> +	 * (altivec support, boot CPU ID, ...)
> +	 */
> +	of_scan_flat_dt(early_init_dt_scan_cpus, NULL);
> +	if (boot_cpuid < 0) {
> +		printk("Failed to identify boot CPU !\n");
> +		BUG();
> +	}
> +
> +	mmu_early_type_finalize();
>  #if defined(CONFIG_FA_DUMP) || defined(CONFIG_PRESERVE_FA_DUMP)
>  	/*
>  	 * If we fail to reserve memory for firmware-assisted dump then
> @@ -884,19 +899,6 @@ void __init early_init_devtree(void *params)
>  	 * FIXME .. and the initrd too? */
>  	move_device_tree();
>  
> -	DBG("Scanning CPUs ...\n");
> -
> -	dt_cpu_ftrs_scan();
> -
> -	/* Retrieve CPU related informations from the flat tree
> -	 * (altivec support, boot CPU ID, ...)
> -	 */
> -	of_scan_flat_dt(early_init_dt_scan_cpus, NULL);
> -	if (boot_cpuid < 0) {
> -		printk("Failed to identify boot CPU !\n");
> -		BUG();
> -	}
> -
>  	save_fscr_to_task();
>  
>  #if defined(CONFIG_SMP) && defined(CONFIG_PPC64)
> diff --git a/arch/powerpc/mm/init_64.c b/arch/powerpc/mm/init_64.c
> index b6f3ae03ca9e..cd52c1baa3bc 100644
> --- a/arch/powerpc/mm/init_64.c
> +++ b/arch/powerpc/mm/init_64.c
> @@ -622,8 +622,10 @@ static void __init early_init_memory_block_size(void)
>  	of_scan_flat_dt(probe_memory_block_size, &memory_block_size);
>  }
>  
> -void __init mmu_early_init_devtree(void)

Let's also add a comment here to be more explicit perhaps. Because it has
caused confusion in past.

/*
 * mmu_early_init_vec5(): For non-hv mode (Pseries LPAR), whether we can do
 * radix or not depend upon hypervisor vec5 values. This functions checks
 * ibm,architecture-vec-5 and updates cur_cpu_spec->mmu_features bits
 * accordingly.
 * After this function returns, early_radix_enabled() can be used
 * to check if radix is supported.
 */
> +
> +void __init mmu_early_type_finalize(void)
>  {
> +
>  	bool hvmode = !!(mfmsr() & MSR_HV);
>  
>  	/* Disable radix mode based on kernel command line. */
> @@ -634,6 +636,20 @@ void __init mmu_early_init_devtree(void)
>  			pr_warn("WARNING: Ignoring cmdline option disable_radix\n");
>  	}
>  
> +	/*
> +	 * Check /chosen/ibm,architecture-vec-5 if running as a guest.
> +	 * When running bare-metal, we can use radix if we like
> +	 * even though the ibm,architecture-vec-5 property created by
> +	 * skiboot doesn't have the necessary bits set.
> +	 */
> +	if (!hvmode)
> +		early_check_vec5();
> +}

-ritesh