[RFC PATCH V3] x86/VMBus: Confidential VMBus for dynamic DMA transfers

[Tianyu Lan <ltykernel@gmail.com>]

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 <mhklinux@outlook.com>
Signed-off-by: Tianyu Lan <tiala@microsoft.com>
---
 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;
+}
+
 static inline bool is_swiotlb_force_bounce(struct device *dev)
 {
 	struct io_tlb_mem *mem = dev->dma_io_tlb_mem;
-- 
2.50.1