Re: [PATCH 1/3] arm64/mm: Simplify SWIOTLB setup in arch_mm_preinit()

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

Mostafa Saleh <smostafa@google.com> writes:

> Hi Mike,
>
> On Wed, Jun 03, 2026 at 03:27:52PM +0300, Mike Rapoport wrote:
>> Hi,
>> 
>> On Wed, Jun 03, 2026 at 11:05:20AM +0000, Mostafa Saleh wrote:
>> > At the moment, arch_mm_preinit() checks if the system has limited
>> > addressing or is running under CCA to enable SWIOTLB, only after to
>> > be forced to true anyway if it was false due to
>> > CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC being unconditionally true for
>> > arm64.
>> > 
>> > Simplify this logic, by making it clear that SWIOTLB is always used
>> > but its size depends on the address layout of the system.
>> > 
>> > Signed-off-by: Mostafa Saleh <smostafa@google.com>
>> > ---
>> >  arch/arm64/mm/init.c | 11 ++++-------
>> >  1 file changed, 4 insertions(+), 7 deletions(-)
>> > 
>> > diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
>> > index 97987f850a33..fcc9c05a8fe6 100644
>> > --- a/arch/arm64/mm/init.c
>> > +++ b/arch/arm64/mm/init.c
>> > @@ -336,25 +336,22 @@ void __init arch_setup_zero_pages(void)
>> >  void __init arch_mm_preinit(void)
>> >  {
>> >  	unsigned int flags = SWIOTLB_VERBOSE;
>> > -	bool swiotlb = max_pfn > PFN_DOWN(arm64_dma_phys_limit);
>> > +	bool limited_addressing = max_pfn > PFN_DOWN(arm64_dma_phys_limit);
>> >  
>> >  	if (is_realm_world()) {
>> > -		swiotlb = true;
>> >  		flags |= SWIOTLB_FORCE;
>> > -	}
>> > -
>> > -	if (IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) && !swiotlb) {
>> > +	} else if (!limited_addressing) {
>> 
>> Can realm_world() coexist with limited_addressing? Then the size adjustment
>> is still required.
>
> My guess would be no, my understanding is that CCA guests have niche
> hardware.
>
> However, the check for limited_addressing is negated, we adjust the
> SWIOTLB to be smaller if no bouncing is needed.
> So, if this is a realm world, address space layout is irrelevant as we
> keep the original size which is larger.
>

That’s correct. Realm guests need swiotlb to always be enabled, and we
should not reduce the size of the pool.

>
> Otherwise, this alters the current behaviour.
>
>> 
>> I'd also make this more explicit:
>> 
>> 	if (max_pfn > PFN_DOWN(arm64_dma_phys_limit))
>
> Sure, I can drop the bool.
>

-aneesh