[PATCH] armv8: mmu: support unmapping regions with set_one_region()

[Casey Connolly]

On 05/05/2026 11:59, Sumit Garg wrote:
> On Mon, May 04, 2026 at 07:53:54PM +0200, Casey Connolly wrote:
>> Currently set_one_region() implicitly assumes that we want to map a
>> region and aggressively splits blocks into tables to do this, but when
>> called with PTE_TYPE_FAULT to unmap a currently mapped region it may
>> try to unnecessarily split blocks which doesn't make sense if the entire
>> block should actually be unmapped. In the end it then has to walk every
>> single page and create a bunch of empty tables.
>>
>> Introduce a check for this kind of behaviour and optimise with a fast
>> path, if we're unmapping a region >= the size of this entry then we can
>> just unmap the entire PTE and whatever it contains.
>>
>> This fixes some bogus empty tables being left behind when carving out
>> reserved memory regions on Qualcomm, and should improve the performance
>> of the break-before-make in mmu_change_region_attr().
>>
>> Signed-off-by: Casey Connolly <casey.connolly at linaro.org>
>> ---
>>  arch/arm/cpu/armv8/cache_v8.c | 16 ++++++++++++++++
>>  1 file changed, 16 insertions(+)
> 
> Nice optimization, FWIW:
> 
> Acked-by: Sumit Garg <sumit.garg at oss.qualcomm.com>
> Tested-by: Sumit Garg <sumit.garg at oss.qualcomm.com>

Could you drop this on the v2 instead? Sorry for the confusion this
version had a silly bug.
> 
> -Sumit
> 
>>
>> diff --git a/arch/arm/cpu/armv8/cache_v8.c b/arch/arm/cpu/armv8/cache_v8.c
>> index 39479df7b21f..b1a1fd571341 100644
>> --- a/arch/arm/cpu/armv8/cache_v8.c
>> +++ b/arch/arm/cpu/armv8/cache_v8.c
>> @@ -939,8 +939,20 @@ static u64 set_one_region(u64 start, u64 size, u64 attrs, bool flag, int level)
>>  	int levelshift = level2shift(level);
>>  	u64 levelsize = 1ULL << levelshift;
>>  	u64 *pte = find_pte(start, level);
>>  
>> +	/*
>> +	 * If we're trying to unmap a region then check if it's already unmapped or if it's bigger
>> +	 * then the PTE we're looking at right now, in the first case we can do nothing and in the
>> +	 * second case we just need to unmap this page/block.
>> +	 * Otherwise we will needlessly create new tables until we have traversed every single page
>> +	 * in the region.
>> +	 */
>> +	if (attrs == PTE_TYPE_FAULT && (pte_type(pte) == PTE_TYPE_FAULT || size >= levelsize)) {
>> +		*pte &= ~PMD_ATTRMASK;
>> +		return levelsize;
>> +	}
>> +
>>  	/* Can we can just modify the current level block PTE? */
>>  	if (is_aligned(start, size, levelsize)) {
>>  		if (flag) {
>>  			*pte &= ~PMD_ATTRMASK;
>> @@ -1081,8 +1093,12 @@ void mmu_change_region_attr(phys_addr_t addr, size_t siz, u64 attrs)
>>  	flush_dcache_range(gd->arch.tlb_addr,
>>  			   gd->arch.tlb_addr + gd->arch.tlb_size);
>>  	__asm_invalidate_tlb_all();
>>  
>> +	/* If we were unmapping a region then we're done! */
>> +	if (attrs == PTE_TYPE_FAULT)
>> +		return;
>> +
>>  	mmu_change_region_attr_nobreak(addr, siz, attrs);
>>  }
>>  
>>  int pgprot_set_attrs(phys_addr_t addr, size_t size, enum pgprot_attrs perm)
>> -- 
>> 2.53.0
>>

-- 
// Casey (she/her)