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Date: Fri, 7 Aug 2026 12:58:29 +0100
From: Will Deacon <will@kernel.org>
To: "Aneesh Kumar K.V (Arm)" <aneesh.kumar@kernel.org>
Cc: iommu@lists.linux.dev, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-coco@lists.linux.dev,
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Subject: Re: [PATCH] arm64: swiotlb: Keep the default size for protected
 guests
Message-ID: <anXIZSh9lq99VG9X@willie-the-truck>
References: <20260717180442.110954-18-aneesh.kumar@kernel.org>
 <20260807092612.2202005-1-aneesh.kumar@kernel.org>
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In-Reply-To: <20260807092612.2202005-1-aneesh.kumar@kernel.org>

On Fri, Aug 07, 2026 at 02:56:12PM +0530, Aneesh Kumar K.V (Arm) wrote:
> Realm guests and protected KVM guests may require swiotlb for device DMA
> even when all system RAM is addressable by the DMA zones.
> 
> Do not reduce the SWIOTLB buffer to the kmalloc-bouncing size for these
> guests, as the reduced number of slots can be exhausted during CCA guest
> operation.
> 
> Fixes: 30c5e45ee2c5 ("dma-direct: make dma_direct_map_phys() honor DMA_ATTR_CC_SHARED")
> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
> ---
>  arch/arm64/mm/init.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index e308a7cabd12..24caaf10e755 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -339,7 +339,8 @@ void __init arch_mm_preinit(void)
>  {
>  	unsigned int flags = SWIOTLB_VERBOSE;
>  
> -	if (max_pfn <= PFN_DOWN(arm64_dma_phys_limit)) {
> +	if (!cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) &&
> +	    max_pfn <= PFN_DOWN(arm64_dma_phys_limit)) {

Protected guests under pKVM rely on restricted DMA, so won't this just
waste memory for them?

Will