On Wed, Mar 25, 2026 at 5:22 PM Leon Romanovsky <[email protected]> wrote:
>
> On Wed, Mar 25, 2026 at 03:56:49AM -0400, Tianyu Lan wrote:
> > Hyper-V provides Confidential VMBus to communicate between
> > device model and device guest driver via encrypted/private
> > memory in Confidential VM. The device model is in OpenHCL
> > (https://openvmm.dev/guide/user_guide/openhcl.html) that
> > plays the paravisor role.
> >
> > For a VMBus device, there are two communication methods to
> > talk with Host/Hypervisor. 1) VMBUS Ring buffer 2) Dynamic
> > DMA transfer.
> >
> > The Confidential VMBus Ring buffer has been upstreamed by
> > Roman Kisel(commit 6802d8af47d1).
> >
> > The dynamic DMA transition of VMBus device normally goes
> > through DMA core and it uses SWIOTLB as bounce buffer in
> > a CoCo VM.
> >
> > The Confidential VMBus device can do DMA directly to
> > private/encrypted memory. Because the swiotlb is decrypted
> > memory, the DMA transfer must not be bounced through the
> > swiotlb, so as to preserve confidentiality. This is different
> > from the default for Linux CoCo VMs, so disable the VMBus
> > device's use of swiotlb.
> >
> > Expose swiotlb_dev_disable() from DMA Core to disable
> > bounce buffer for device.
>
> It feels awkward and like a layering violation to let arbitrary kernel
> drivers manipulate SWIOTLB, which sits beneath the DMA core.
>
Hi Leon:
Thanks for your review. I will try other way since now DMA core has
not stand way to disable device swiotlb.
--
Thanks
Tianyu Lan
