Re: [PATCH v2 3/8] ARM: mmu: use reserve mem entries to modify maps

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

On 03.08.21 11:44, Rouven Czerwinski wrote:
> Use the information from the reserved memory entries to modify the
> mapping of memory regions to mark them as uncachable and not-executable.
> This also prevents the processor from speculating into these regions,
> preventing hard to debug scenarios where boots fail for unknown reasons.
> 
> Signed-off-by: Rouven Czerwinski <r.czerwinski@pengutronix.de>
> ---
>  arch/arm/cpu/mmu.c | 34 ++++++++++++++++++++++++++++++++--
>  1 file changed, 32 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/cpu/mmu.c b/arch/arm/cpu/mmu.c
> index 6388e1bf14..73f4cf5b36 100644
> --- a/arch/arm/cpu/mmu.c
> +++ b/arch/arm/cpu/mmu.c
> @@ -17,6 +17,7 @@
>  #include <memory.h>
>  #include <asm/system_info.h>
>  #include <asm/sections.h>
> +#include <of.h>
>  
>  #include "mmu.h"
>  
> @@ -407,6 +408,36 @@ static void vectors_init(void)
>  	create_vector_table(ARM_LOW_VECTORS);
>  }
>  
> +static void create_sections_with_intersect(struct memory_bank *bank)
> +{
> +	struct of_reserve_map *res_map;
> +	unsigned long j_end;
> +	unsigned long end;
> +	unsigned long j;
> +	u32 pmd_flags;
> +	int i;
> +
> +	res_map = of_get_reserve_map();
> +	if (!res_map)
> +		return;

If there is no reserve map, you should still map the banks cached.
So this early exit is wrong.

> +
> +	end = bank->start + bank->size - 1;
> +
> +	for (j = bank->start; j < end; j += PGDIR_SIZE) {
> +		pmd_flags = PMD_SECT_DEF_CACHED;
> +		j_end = j + PGDIR_SIZE - 1;
> +
> +		for (i = 0; i < res_map->num_entries; i++) {
> +			if ((BIT(i) & res_map->xn) &&
> +			    j_end >= res_map->start[i] &&
> +			    j_end <= res_map->end[i])
> +				pmd_flags = PMD_SECT_DEF_UNCACHED | PMD_SECT_XN;
> +		}
> +
> +		create_sections(ttb, j, j_end, pmd_flags);
> +	}

Do we need the nested loop? Can't we create sections cached first
and then iterate once over the reserved map, deduce the sections containing
start and end and map those uncached?

See map_region in mmy-early.c for an example.
Am I missing something?

> +}
> +
>  /*
>   * Prepare MMU for usage enable it.
>   */
> @@ -468,8 +499,7 @@ void __mmu_init(bool mmu_on)
>  	vectors_init();
>  
>  	for_each_memory_bank(bank) {
> -		create_sections(ttb, bank->start, bank->start + bank->size - 1,
> -				PMD_SECT_DEF_CACHED);
> +		create_sections_with_intersect(bank);
>  		__mmu_cache_flush();
>  	}
>  
> 

Cheers,
Ahmad


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