Re: [PATCH 1/1] arm64: realm: Use aliased addresses for device DMA to shared buffers

[Suzuki K Poulose <suzuki.poulose@arm.com>]

On 15/02/2025 14:38, Aneesh Kumar K.V wrote:
> Aneesh Kumar K.V <aneesh.kumar@kernel.org> writes:
> 
>> Suzuki K Poulose <suzuki.poulose@arm.com> writes:
>>
>>> 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.
>>>
>>> Add the support for this by providing arm64 version of the phys_to_dma(). We
>>> cannot use the __sme_mask as it assumes the "encryption" always "sets a bit",
>>> which is the opposite for CCA. i.e., "set a bit" for "decrypted" address. So,
>>> move the common code that can be reused by all - i.e., add __phys_to_dma() and
>>> __dma_to_phys() - and do the arch specific processing.
>>>
>>> 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@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: Tom Lendacky <thomas.lendacky@amd.com>
>>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
>>> ---
>>>   arch/arm64/Kconfig                  |  1 +
>>>   arch/arm64/include/asm/dma-direct.h | 38 +++++++++++++++++++++++++++++
>>>   include/linux/dma-direct.h          | 35 +++++++++++++++++---------
>>>   3 files changed, 62 insertions(+), 12 deletions(-)
>>>   create mode 100644 arch/arm64/include/asm/dma-direct.h
>>>
>>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>>> index fcdd0ed3eca8..7befe04106de 100644
>>> --- a/arch/arm64/Kconfig
>>> +++ b/arch/arm64/Kconfig
>>> @@ -41,6 +41,7 @@ config ARM64
>>>   	select ARCH_HAS_NMI_SAFE_THIS_CPU_OPS
>>>   	select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
>>>   	select ARCH_HAS_NONLEAF_PMD_YOUNG if ARM64_HAFT
>>> +	select ARCH_HAS_PHYS_TO_DMA
>>>   	select ARCH_HAS_PTE_DEVMAP
>>>   	select ARCH_HAS_PTE_SPECIAL
>>>   	select ARCH_HAS_HW_PTE_YOUNG
>>> diff --git a/arch/arm64/include/asm/dma-direct.h b/arch/arm64/include/asm/dma-direct.h
>>> new file mode 100644
>>> index 000000000000..37c3270542b8
>>> --- /dev/null
>>> +++ b/arch/arm64/include/asm/dma-direct.h
>>> @@ -0,0 +1,38 @@
>>> +/* SPDX-License-Identifier: GPL-2.0-only */
>>> +#ifndef __ASM_DMA_DIRECT_H
>>> +#define __ASM_DMA_DIRECT_H
>>> +
>>> +#include <asm/pgtable-prot.h>
>>> +
>>> +static inline unsigned long addr_to_shared(unsigned long addr)
>>> +{
>>> +	if (is_realm_world())
>>> +		addr |= prot_ns_shared;
>>> +	return addr;
>>> +}
>>> +
>>> +static inline unsigned long addr_to_private(unsigned long addr)
>>> +{
>>> +	if (is_realm_world())
>>> +		addr &= prot_ns_shared - 1;
>>> +	return addr;
>>> +}
>>> +
>>> +static inline dma_addr_t phys_to_dma(struct device *dev, phys_addr_t paddr)
>>> +{
>>> +	return __phys_to_dma(dev, paddr);
>>> +}
>>> +
>>> +static inline dma_addr_t phys_to_dma_unencrypted(struct device *dev,
>>> +						 phys_addr_t paddr)
>>> +{
>>> +	return addr_to_shared(__phys_to_dma(dev, paddr));
>>> +}
>>> +#define phys_to_dma_unencrypted phys_to_dma_unencrypted
>>> +
>>> +static inline phys_addr_t dma_to_phys(struct device *dev, dma_addr_t dma_addr)
>>> +{
>>> +	return addr_to_private(__dma_to_phys(dev, dma_addr));
>>> +}
>>> +
>>> +#endif	/* __ASM_DMA_DIRECT_H */
>>> diff --git a/include/linux/dma-direct.h b/include/linux/dma-direct.h
>>> index d7e30d4f7503..3e9bf6ca640e 100644
>>> --- a/include/linux/dma-direct.h
>>> +++ b/include/linux/dma-direct.h
>>> @@ -72,18 +72,36 @@ static inline dma_addr_t dma_range_map_max(const struct bus_dma_region *map)
>>>   	return ret;
>>>   }
>>>   
>>> +static inline dma_addr_t __phys_to_dma(struct device *dev, phys_addr_t paddr)
>>> +{
>>> +	if (dev->dma_range_map)
>>> +		return translate_phys_to_dma(dev, paddr);
>>> +	return paddr;
>>> +}
>>> +
>>> +static inline phys_addr_t __dma_to_phys(struct device *dev, dma_addr_t dma_addr)
>>> +{
>>> +	phys_addr_t paddr;
>>> +
>>> +	if (dev->dma_range_map)
>>> +		paddr = translate_dma_to_phys(dev, dma_addr);
>>> +	else
>>> +		paddr = dma_addr;
>>> +
>>> +	return paddr;
>>> +}
>>> +
>>>   #ifdef CONFIG_ARCH_HAS_PHYS_TO_DMA
>>>   #include <asm/dma-direct.h>
>>>   #ifndef phys_to_dma_unencrypted
>>>   #define phys_to_dma_unencrypted		phys_to_dma
>>>   #endif
>>>   #else
>>> +
>>>   static inline dma_addr_t phys_to_dma_unencrypted(struct device *dev,
>>>   		phys_addr_t paddr)
>>>   {
>>> -	if (dev->dma_range_map)
>>> -		return translate_phys_to_dma(dev, paddr);
>>> -	return paddr;
>>> +	return __phys_to_dma(dev, paddr);
>>>   }
>>>   
>>>   /*
>>> @@ -94,19 +112,12 @@ static inline dma_addr_t phys_to_dma_unencrypted(struct device *dev,
>>>    */
>>>   static inline dma_addr_t phys_to_dma(struct device *dev, phys_addr_t paddr)
>>>   {
>>> -	return __sme_set(phys_to_dma_unencrypted(dev, paddr));
>>> +	return __sme_set(__phys_to_dma(dev, paddr));
>>>   }
>>>   
>>>   static inline phys_addr_t dma_to_phys(struct device *dev, dma_addr_t dma_addr)
>>>   {
>>> -	phys_addr_t paddr;
>>> -
>>> -	if (dev->dma_range_map)
>>> -		paddr = translate_dma_to_phys(dev, dma_addr);
>>> -	else
>>> -		paddr = dma_addr;
>>> -
>>> -	return __sme_clr(paddr);
>>> +	return __sme_clr(__dma_to_phys(dev, dma_addr));
>>>   }
>>>   #endif /* !CONFIG_ARCH_HAS_PHYS_TO_DMA */
>>>   
>>> -- 
>>> 2.43.0
>>
>> How about the below?
>>
>> The function name addr_to_shared is too generic to be included in the
>> dma-direct.h header file. Since we don’t expect it to be called
>> directly, we can either inline it or find a more specific name.
>>
>> Additionally, for dma_to_phys conversion, we first retrieve the private
>> address/alias before switching to the physical address. While both
>> approaches yield the correct result, this change more clearly defines the
>> conversion rules?
>>
> 
> Also translate_dma_to_phys/translate_phys_to_dma work with private dma
> addr/alias right?
> 

You're right, that needs to be fixed. Luckily (hopefully) DMA ranges
are not used by AMD SME.

Suzuki


>>
>> modified   arch/arm64/include/asm/dma-direct.h
>> @@ -4,14 +4,14 @@
>>   
>>   #include <asm/pgtable-prot.h>
>>   
>> -static inline unsigned long addr_to_shared(unsigned long addr)
>> +static inline unsigned long shared_dma_addr(unsigned long addr)
>>   {
>>   	if (is_realm_world())
>>   		addr |= prot_ns_shared;
>>   	return addr;
>>   }
>>   
>> -static inline unsigned long addr_to_private(unsigned long addr)
>> +static inline unsigned long private_dma_addr(unsigned long addr)
>>   {
>>   	if (is_realm_world())
>>   		addr &= prot_ns_shared - 1;
>> @@ -26,13 +26,14 @@ static inline dma_addr_t phys_to_dma(struct device *dev, phys_addr_t paddr)
>>   static inline dma_addr_t phys_to_dma_unencrypted(struct device *dev,
>>   						 phys_addr_t paddr)
>>   {
>> -	return addr_to_shared(__phys_to_dma(dev, paddr));
>> +	return shared_dma_addr(__phys_to_dma(dev, paddr));
>>   }
>>   #define phys_to_dma_unencrypted phys_to_dma_unencrypted
>>   
>>   static inline phys_addr_t dma_to_phys(struct device *dev, dma_addr_t dma_addr)
>>   {
>> -	return addr_to_private(__dma_to_phys(dev, dma_addr));
>> +	/* if it is a shared dma addr convert to private before looking for phys_addr */
>> +	return __dma_to_phys(dev, private_dma_addr(dma_addr));
>>   }
>>   
>>   #endif	/* __ASM_DMA_DIRECT_H */