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

From: Mostafa Saleh <[email-masked]>
Date: Wed, 3 Jun 2026 13:13:49 +0000

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 <[email-masked]>
> > ---
> >  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.

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.

Thanks,
Mostafa

> 
> >  		/*
> >  		 * If no bouncing needed for ZONE_DMA, reduce the swiotlb
> >  		 * buffer for kmalloc() bouncing to 1MB per 1GB of RAM.
> >  		 */
> >  		unsigned long size =
> >  			DIV_ROUND_UP(memblock_phys_mem_size(), 1024);
> > +
> >  		swiotlb_adjust_size(min(swiotlb_size_or_default(), size));
> > -		swiotlb = true;
> >  	}
> >  
> > -	swiotlb_init(swiotlb, flags);
> > +	swiotlb_init(true, flags);
> >  
> >  	/*
> >  	 * Check boundaries twice: Some fundamental inconsistencies can be
> > -- 
> > 2.54.0.1032.g2f8565e1d1-goog
> > 
> 
> -- 
> Sincerely yours,
> Mike.