On Fri, Mar 27, 2026 at 1:05 AM Easwar Hariharan
<[email protected]> wrote:
>
> On 3/25/2026 12:56 AM, 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.
> >
> > Suggested-by: Michael Kelley <[email protected]>
> > Signed-off-by: Tianyu Lan <[email protected]>
> > ---
> > drivers/hv/vmbus_drv.c | 6 +++++-
> > include/linux/swiotlb.h | 5 +++++
> > 2 files changed, 10 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> > index 3d1a58b667db..84e6971fc90f 100644
> > --- a/drivers/hv/vmbus_drv.c
> > +++ b/drivers/hv/vmbus_drv.c
> > @@ -2184,11 +2184,15 @@ int vmbus_device_register(struct hv_device
> > *child_device_obj)
> > child_device_obj->device.dma_mask = &child_device_obj->dma_mask;
> > dma_set_mask(&child_device_obj->device, DMA_BIT_MASK(64));
> >
> > + device_initialize(&child_device_obj->device);
> > + if (child_device_obj->channel->co_external_memory)
> > + swiotlb_dev_disable(&child_device_obj->device);
> > +
> > /*
> > * Register with the LDM. This will kick off the driver/device
> > * binding...which will eventually call vmbus_match() and
> > vmbus_probe()
> > */
> > - ret = device_register(&child_device_obj->device);
> > + ret = device_add(&child_device_obj->device);
> > if (ret) {
> > pr_err("Unable to register child device\n");
> > put_device(&child_device_obj->device);
> > diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> > index 3dae0f592063..7c572570d5d9 100644
> > --- a/include/linux/swiotlb.h
> > +++ b/include/linux/swiotlb.h
> > @@ -169,6 +169,11 @@ static inline struct io_tlb_pool
> > *swiotlb_find_pool(struct device *dev,
> > return NULL;
> > }
> >
> > +static inline bool swiotlb_dev_disable(struct device *dev)
> > +{
> > + return dev->dma_io_tlb_mem == NULL;
>
> Is there an extra = here?
>
> - Easwar (he/him)
Hi Easwar:
Thanks for your review. Nice catch. Oops. Will try other way to disable
device bounce buffer in the next version.
--
Thanks
Tianyu Lan
