Re: [PATCH v3 0/3] arm64: realm: Fix DMA address for devices

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

On 04/03/2025 13:40, Marek Szyprowski wrote:
> Hi,
> 
> On 03.03.2025 12:35, Suzuki K Poulose wrote:
>> On 27/02/2025 14:41, Suzuki K Poulose wrote:
>>> Linux can be run as a Confidential Guest in Arm CCA from Linux v6.13.
>>> The address
>>> space (GPA or IPA) of a Realm VM is split into two halves, with
>>> private bottom
>>> half and shared top half. In Linux we treat the "top" bit of the IPA
>>> space as
>>> an attribute, to indicate whether it is shared or not (MSB == 1
>>> implies shared).
>>> Stage2 (GPA to PA) translations used by the CPU accesses, cover the
>>> full IPA space,
>>> and are managed by RMM. The "top" bit as attribute is only a software
>>> construct.
>>>
>>> At present any device passed through to a Realm is treated as
>>> untrusted and the
>>> Realm uses bounce buffering for any DMA, using the "decrypted"
>>> (shared) DMA
>>> buffers (i.e., IPA with top bit set). In Linux, we only send the
>>> "DMA" address
>>> masking the "top" bit. In Arm CCA, SMMU for untrusted devices are
>>> managed by the
>>> non-secure Host and thus it can be confusing for the host/device when
>>> an unmasked
>>> address is provided. Given there could be other hypervisors than
>>> Linux/KVM
>>> running Arm CCA guests, the Realm Guest must adhere to a single
>>> convention for
>>> the DMA address. This gets further complicated when we add support
>>> for trusted
>>> devices, which can DMA into the full Realm memory space, once
>>> accepted. Thus,
>>> a DMA masked address (with "top" bit lost) will prevent a trusted
>>> device from
>>> accessing a shared buffer.
>>>
>>> To resolve this Arm has decided to standardise the DMA address used
>>> by the Realm
>>> to include the full IPA address bits (including the "top" bit, which
>>> Linux uses
>>> as an attribute). This implies, any DMA to a shared buffer must have
>>> the top bit
>>> of the IPA space set.
>>>
>>> There is already a provision to do this in phys_to_dma* and
>>> dma_to_phys(), but
>>> that is specific to AMD SME and is quite the opposite of what we need
>>> for Arm CCA.
>>> i.e., For Arm CCA we need to set the bit for "decrypted" DMA and
>>> clear the bit
>>> for "encrypted".
>>>
>>> This series converts the existing __sme_* helpers to a bit more
>>> generalised versions :
>>> dma_addr_decrypted() and dma_encrypted(). Also, while converting a
>>> DMA address back
>>> to CPU physical address requires clearing off any
>>> "encryption/decryption" bits.
>>> I have named this "dma_addr_canonical()". (The other options are :
>>>     * dma_addr_clear_encryption - Well, not just for encryption, but
>>> we clear decryption
>>>       too, so not ideal.
>>>     * dma_addr_normal
>>>     * dma_addr_clear
>>>     * dma_addr_default
>>>
>>> This also implies that the VMMs must take care to :
>>>
>>>    1. Create the S2-SMMU mappings for VFIO at the "unprotected" alias.
>>>    2. Always mask the "top" bit off any IPA it receives from the Realm
>>> for DMA.
>>>       KVM already does that today and no changes are required.
>>>
>>> A kvmtool branch with the changes above is available here [1]. There
>>> are two
>>> patches [2] & [3], that are really required on top of the Arm CCA
>>> support.
>>>
>>> Ideally it would be good to get this backported to v6.13 stable
>>> kernel releases
>>> to make sure that they are compliant with this change.
>>>
>>
>> Please could you take a look at this series and let us know your
>> thoughts ? If you are happy with the changes, are you happy to pull
>> this through the DMA MAP tree ? The relevant bits have been reviewed/
>> acked by people (arm64 and AMD bits).
> 
> The changes look fine. However I won't be able to setup new dma-mapping
> git tree this week because I got really sick has to stay in bed. :/ If
> You don't want such delay, please merge it via ARM64 tree. Here is my:

Sorry to hear that. Hope you feel better soon.

> 
> Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>

Thanks Marek.

Btw, this series fixes the "Realm Guest" support for Linux, which was 
merged in v6.13. To be precise, this should have :

Fixes: 42be24a4178f ("arm64: Enable memory encrypt for Realms")

Will/Catalin,

Please let me know if you would like me to send the series with
all the Acks, Reviews and mainly the Fixes tag added ?

Kind regards
Suzuki

> 
> 
> Best regards