Re: [PATCH v3 3/3] arm64: realm: Use aliased addresses for device DMA to shared buffers

[Robin Murphy <robin.murphy@arm.com>]

On 27/02/2025 2:41 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_addr_{encrypted, canonical}
> 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.

FWIW,

Reviewed-by: Robin Murphy <robin.murphy@arm.com>

> Cc: Will Deacon <will@kernel.org>
> Cc: Jean-Philippe Brucker <jean-philippe@linaro.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Robin Murphy <robin.murphy@arm.com>
> Cc: Steven Price <steven.price@arm.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Cc: Aneesh Kumar K.V <aneesh.kumar@kernel.org>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> ---
> Changes since v2:
>   - Drop dma_addr_encrypted() helper, which is a NOP for CCA ( Aneesh )
>   - Only mask the "top" IPA bit and not all the bits beyond top bit. ( Robin )
>   - Use PROT_NS_SHARED, now that we only set/clear top bit. (Gavin)
> ---
>   arch/arm64/include/asm/mem_encrypt.h | 11 +++++++++++
>   1 file changed, 11 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/mem_encrypt.h b/arch/arm64/include/asm/mem_encrypt.h
> index f8f78f622dd2..a2a1eeb36d4b 100644
> --- a/arch/arm64/include/asm/mem_encrypt.h
> +++ b/arch/arm64/include/asm/mem_encrypt.h
> @@ -21,4 +21,15 @@ static inline bool force_dma_unencrypted(struct device *dev)
>   	return is_realm_world();
>   }
>   
> +/*
> + * For Arm CCA guests, canonical addresses are "encrypted", so no changes
> + * required for dma_addr_encrypted().
> + * The unencrypted DMA buffers must be accessed via the unprotected IPA,
> + * "top IPA bit" set.
> + */
> +#define dma_addr_unencrypted(x)		((x) | PROT_NS_SHARED)
> +
> +/* Clear the "top" IPA bit while converting back */
> +#define dma_addr_canonical(x)		((x) & ~PROT_NS_SHARED)
> +
>   #endif	/* __ASM_MEM_ENCRYPT_H */