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

From: Mike Rapoport <[email-masked]>
Date: Wed, 3 Jun 2026 15:27:52 +0300

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.

I'd also make this more explicit:

	if (max_pfn > PFN_DOWN(arm64_dma_phys_limit))

>  		/*
>  		 * 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.