Tianyu Lan <[email protected]> writes: > On Mon, Aug 3, 2026 at 5:04 PM Aneesh Kumar K.V <[email protected]> > 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 “dma-mapping: Use > DMA_ATTR_CC_SHARED”（https://lkml.org/lkml/2026/6/4/588). This 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(). >
Can we rework this patch to use that? > >> 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. If we update force_dma_unencrypted(dev) to return false when an encrypted memory address is required, wouldn't the existing DMA-direct support handle this case? Or are there issues with the existing code? If so, could you explain them in more detail? -aneesh
