linux-kernel - Re: [PATCH v2 3/3] arm64: realm: Use aliased addresses for device DMA to shared buffers

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Message-ID: <0b0705aa-8c85-4502-8450-a6c7fdbdbdbd@arm.com>
Date: Tue, 25 Feb 2025 13:04:35 +0000
From: Robin Murphy <robin.murphy@....com>
To: Suzuki K Poulose <suzuki.poulose@....com>, will@...nel.org,
 catalin.marinas@....com
Cc: maz@...nel.org, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
 aneesh.kumar@...nel.org, steven.price@....com,
 Jean-Philippe Brucker <jean-philippe@...aro.org>,
 Christoph Hellwig <hch@....de>, Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v2 3/3] arm64: realm: Use aliased addresses for device DMA
 to shared buffers

On 2025-02-19 10:07 pm, Suzuki K Poulose wrote:
> When a device performs DMA to a shared buffer using physical addresses,
> (without Stage1 translation), the device must use the "{I}PA address" with the
> top bit set in Realm. This is to make sure that a trusted device will be able
> to write to shared buffers as well as the protected buffers. Thus, a Realm must
> always program the full address including the "protection" bit, like AMD SME
> encryption bits.
> 
> Enable this by providing arm64 specific dma_{encrypted,decrypted,clear_encryption}
> helpers for Realms. Please note that the VMM needs to similarly make sure that
> the SMMU Stage2 in the Non-secure world is setup accordingly to map IPA at the
> unprotected alias.
> 
> Cc: Will Deacon <will@...nel.org>
> Cc: Jean-Philippe Brucker <jean-philippe@...aro.org>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Robin Murphy <robin.murphy@....com>
> Cc: Steven Price <steven.price@....com>
> Cc: Christoph Hellwig <hch@....de>
> Cc: Tom Lendacky <thomas.lendacky@....com>
> Cc: Aneesh Kumar K.V <aneesh.kumar@...nel.org>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
> ---
>   arch/arm64/include/asm/mem_encrypt.h | 22 ++++++++++++++++++++++
>   1 file changed, 22 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/mem_encrypt.h b/arch/arm64/include/asm/mem_encrypt.h
> index f8f78f622dd2..aeda3bba255e 100644
> --- a/arch/arm64/include/asm/mem_encrypt.h
> +++ b/arch/arm64/include/asm/mem_encrypt.h
> @@ -21,4 +21,26 @@ static inline bool force_dma_unencrypted(struct device *dev)
>   	return is_realm_world();
>   }
>   
> +static inline dma_addr_t dma_decrypted(dma_addr_t daddr)
> +{
> +	if (is_realm_world())
> +		daddr |= prot_ns_shared;
> +	return daddr;
> +}
> +#define dma_decrypted dma_decrypted
> +
> +static inline dma_addr_t dma_encrypted(dma_addr_t daddr)
> +{
> +	if (is_realm_world())
> +		daddr &= prot_ns_shared - 1;

Nit: is there a reason this isn't the direct inverse of the other 
operation, i.e. "daddr &= ~prot_ns_shared"? If so, it might be worth 
dropping a comment why we're doing slightly unintuitive arithmetic on a 
pagetable attribute (and if not then maybe just do the more obvious 
thing). I doubt anyone's in a rush to support TBI for DMA, and this 
would be far from the only potential hiccup for that, but still... :)

Thanks,
Robin.

> +	return daddr;
> +}
> +#define dma_encrypted dma_encrypted
> +
> +static inline dma_addr_t dma_clear_encryption(dma_addr_t daddr)
> +{
> +	return dma_encrypted(daddr);
> +}
> +#define dma_clear_encryption dma_clear_encryption
> +
>   #endif	/* __ASM_MEM_ENCRYPT_H */