linux-kernel - Re: [PATCH v2 1/4] swiotlb: dma: its: Enforce host page-size alignment for shared buffers

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Message-ID: <20260106011636.GQ125261@ziepe.ca>
Date: Mon, 5 Jan 2026 21:16:36 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: "Aneesh Kumar K.V (Arm)" <aneesh.kumar@...nel.org>
Cc: linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
	linux-coco@...ts.linux.dev,
	Catalin Marinas <catalin.marinas@....com>, will@...nel.org,
	maz@...nel.org, tglx@...utronix.de, robin.murphy@....com,
	suzuki.poulose@....com, akpm@...ux-foundation.org,
	steven.price@....com
Subject: Re: [PATCH v2 1/4] swiotlb: dma: its: Enforce host page-size
 alignment for shared buffers

On Sun, Dec 21, 2025 at 09:39:17PM +0530, Aneesh Kumar K.V (Arm) wrote:
> +#define mem_encrypt_align mem_encrypt_align
> +static inline size_t mem_encrypt_align(size_t size)
> +{
> +	return size;
> +}
> +

IMHO this is the wrong API.

The issue here is not about alignment, it is about the permitted
granule size for shared/private.

On X86 this will be PAGE_SIZE on ARM64 it is
  max(hypervisor_page_size, PAGE_SIZE)

So think the arch helper should simply be

  __pure size_T mem_encrypt_granule_size(void);

> +	if (WARN_ON(!IS_ALIGNED(addr, mem_encrypt_align(PAGE_SIZE))))
> +		return 0;
> +
> +	if (WARN_ON(!IS_ALIGNED(numpages << PAGE_SHIFT, mem_encrypt_align(PAGE_SIZE))))
> +		return 0;

And then we don't end up with weiro reading stuff like this..

if (WARN_ON(!IS_ALIGNED(addr, mem_encrypt_granule_size())) ||
    WARN_ON(!IS_ALIGNED(numpages, mem_encrypt_granule_size() / PAGE_SIZE)))

Is much more readable..
> @@ -319,8 +319,8 @@ static void __init *swiotlb_memblock_alloc(unsigned long nslabs,
>  		unsigned int flags,
>  		int (*remap)(void *tlb, unsigned long nslabs))
>  {
> -	size_t bytes = PAGE_ALIGN(nslabs << IO_TLB_SHIFT);
>  	void *tlb;
> +	size_t bytes = mem_encrypt_align(nslabs << IO_TLB_SHIFT);

The stuff like this is just ALING(nslabs << IO_TLB_SHIFT, mem_encrypt_granule_size())

etc

Jason