Re: [PATCH 4/6] arm64: use hw_pte_t for fixmap HW PTEs

["David Hildenbrand (Arm)" <david@kernel.org>]

On 9/18/26 18:03, Ryan Roberts wrote:
> On 14/09/2026 14:51, Muhammad Usama Anjum wrote:
>> fixmap_pte() returns a pointer into bm_pte, and early_fixmap_init_pte()
>> installs those arrays as page tables. Their elements are therefore
>> HW PTEs.
>>
>> Change the element type of bm_pte to hw_pte_t so it matches the HW PTE
>> pointers returned and passed to the accessors. This is needed before
>> ARCH_HAS_HW_PTE_T makes HW PTEs and SW PTE values distinct types; the
>> array dimensions and placement are unchanged.
>>
>> Signed-off-by: Muhammad Usama Anjum <usama.anjum@arm.com>
>> ---
>>  arch/arm64/mm/fixmap.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/mm/fixmap.c b/arch/arm64/mm/fixmap.c
>> index 237a9136bc73b..709a97fe327d8 100644
>> --- a/arch/arm64/mm/fixmap.c
>> +++ b/arch/arm64/mm/fixmap.c
>> @@ -31,7 +31,7 @@ static_assert(NR_BM_PMD_TABLES == 1);
>>  
>>  #define BM_PTE_TABLE_IDX(addr)	__BM_TABLE_IDX(addr, PMD_SHIFT)
>>  
>> -static pte_t bm_pte[NR_BM_PTE_TABLES][PTRS_PER_PTE] __bss_pgtbl;
>> +static hw_pte_t bm_pte[NR_BM_PTE_TABLES][PTRS_PER_PTE] __bss_pgtbl;
> 
> I think in my original proposal it was impossible to have a hw_pte value; only a
> hw_pte pointer was possible. Being able to create hw_pte values means that it is
> possible that a hw_pte_t pointer is not actually pointing to an entry in a HW
> pgtable. The main motivation for this is that we want to dereference neighbours
> of a hw pte based on it's pointer and be confident that it is safe. I think this
> removes some of the safety.

If you follow some of the related discussions on the core-mm series, there is a
path forward to go further into that direction.

People instantiating a hw_pte_t on the stack are not really a concern right now.

-- 
Cheers,

David