Drivers: hv: vmbus: Set DMA coherent mask for VMBus devices

In current code, the coherent_dma_mask for VMBus devices is not set, so
it has the default value of 0, which essentially means "invalid". Because
drivers for VMBus devices do not use dma_alloc_*() functions, the usual
use of the coherent mask does not occur, and no errors result.

However, a valid coherent_dma_mask may be needed even though the drivers
don't use dma_alloc_*() functions. In a CoCo VM, the VMBus storvsc and
netvsc drivers must bounce buffer DMA operations through the swiotlb
because the Hyper-V host can't DMA into encrypted guest memory. If the
kernel is built with CONFIG_SWIOTLB_DYNAMIC and the initial swiotlb size
is small, swiotlb code may need to grow the swiotlb in response to a DMA
mapping request. That growth first allocates a transient pool while the
swiotlb is expanded in the background. The transient pool memory is
allocated from the DMA atomic pools, and the allocation code checks for
a valid coherent_dma_mask. With current code, this check fails, then the
DMA mapping request from the storvsc or netvsc driver fails, and finally
an I/O error occurs.

Fix this problem by setting coherent_dma_mask for VMBus devices at the
same time that dma_mask is set. Being a synthetic bus, VMBus does not
have any restrictions on coherent DMA, so the coherent mask is set to
the full 64 bits for all VMBus devices, just like with dma_mask.

Signed-off-by: Michael Kelley <mhklinux@outlook.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>

diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index 23206640c613974a05fb9cddaa7d3dfcd4379793..a6b9a33db65789a4bb5e4484e1814faa33100b0b 100644 (file)
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -2169,6 +2169,7 @@ int vmbus_device_register(struct hv_device *child_device_obj)
        child_device_obj->device.dma_parms = &child_device_obj->dma_parms;
        child_device_obj->device.dma_mask = &child_device_obj->dma_mask;
        dma_set_mask(&child_device_obj->device, DMA_BIT_MASK(64));
+       dma_set_coherent_mask(&child_device_obj->device, DMA_BIT_MASK(64));
 
        /*
         * Register with the LDM. This will kick off the driver/device