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

On Mon, Aug 3, 2026 at 5:04=E2=80=AFPM Aneesh Kumar K.V <aneesh.kumar@kerne=
l.org> wrote:
>
> Tianyu Lan <[email protected]> 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)?
>

Hi Aneesh:
      Thanks for your review. I go through your patchset =E2=80=9Cdma-mappi=
ng: Use
DMA_ATTR_CC_SHARED=E2=80=9D=EF=BC=88https://lkml.org/lkml/2026/6/4/588). Th=
is patch
is compatible with your DMA_ATTR_CC_SHARED attr work in dma-mapping
layer. Once dma ops callbacks get DMA_ATTR_CC_SHARED attr flag and
it may use the flag to replace hyperv_private_memory_dma().

> Looking at functions such as hyperv_dma_alloc_coherent(), how does this
> differ from dma_direct_alloc()?
>
hyperv_dma_alloc_coherent() is to allocate memory and returns encrypted
memory address directly when hyperv_private_memory_dma() is ture.
This change is to keep all changes under Hyper-V subsystem and it's
also compatible with existing solution which changes DMA core with minor
changes.
--
Thanks


Tianyu Lan