Re: [RFC PATCH 1/1] x86/VMBus: DMA transfer with encrypted memory in Coco VM

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

Tianyu Lan <ltykernel@gmail.com> writes:

> In CoCo VMs, system memory is encrypted by default.
> Device drivers typically rely on the DMA core's
> SWIOTLB as a bounce buffer for DMA operations, providing
> decrypted memory that can be shared between the guest and
> host.
>
> For PCI devices with T-Disp support and Confidential
> VMBus devices (https://lkml.org/lkml/2026/7/27/1733) can
> perform DMA transfers directly with private/encrypted
> memory in a CoCo VM.
>
> To support DMA transfer with encrypted memory, Hyper-V
> DMA ops are introduced and bypass some API which may
> use swiotlb as bounce buffer.
>
> The DMA ops used is global data structure(see get_arch_
> dma_ops() and get_dma_ops() for details). There is no
> need to set up for each device individually.
>

Can we use __DMA_ATTR_ALLOC_CC_SHARED instead of checking
hyperv_private_memory_dma(dev) directly? Also, would it be possible to
encapsulate this logic in something like force_dma_encrypted(dev)?

Looking at functions such as hyperv_dma_alloc_coherent(), how does this
differ from dma_direct_alloc()? 

-aneesh