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

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Message-ID: <308f8a08-a6f8-403e-a130-18c7d3db2e23@redhat.com>
Date: Tue, 25 Feb 2025 15:28:24 +1000
From: Gavin Shan <gshan@...hat.com>
To: Suzuki K Poulose <suzuki.poulose@....com>, will@...nel.org,
 robin.murphy@....com, 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 2/25/25 3:24 PM, Gavin Shan wrote: 
> On 2/20/25 8:07 AM, 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
>> +
> 
> There is an existing macro (PROT_NS_SHARED), which is preferred to return
> prot_ns_shared or 0 depending on the availability of the realm capability.
> However, that macro needs to be improved a bit so that it can be used here.
> We need to return 0UL to match with the type of prot_ns_shared (unsigned long)
> 
> -#define PROT_NS_SHARED         (is_realm_world() ? prot_ns_shared : 0)
> +#define PROT_NS_SHARED         (is_realm_world() ? prot_ns_shared : 0UL)
> 
> After that, the chunk of code can be as below.
> 
>      return daddr | PROT_NS_SHARED;
> 
>> +static inline dma_addr_t dma_encrypted(dma_addr_t daddr)
>> +{
>> +    if (is_realm_world())
>> +        daddr &= prot_ns_shared - 1;
>> +    return daddr;
>> +}
>> +#define dma_encrypted dma_encrypted
>> +
> 
> With PROT_NS_SHARED, it can become something like below. (PROT_NS_SHARED - 1)
> is equivalent to -1UL, 'daddr & -1UL' should be fine since it does nothing.
> 

I meant (PROT_NS_SHARED - 1) is equivalent to -1UL when no realm capability
is around :)

>      return daddr & (PROT_NS_SHARED - 1);
> 
>> +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 */

Thanks,
Gavin