From: Michael Kelley <[email protected]> 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 <[email protected]> --- I have not provided a Fixes: tag because the scenario under which the error occurs is an artificial test case that I came across while stressing a unrelated patch set. The fix is valid for general goodness, but the likelihood of the problem occurring in the real world is extremely small. So I see little value in adding this patch to the stable kernel maintainers' workload. If someone wants to argue otherwise, I have no fundamental objection to adding the Fixes: tag. drivers/hv/vmbus_drv.c | 1 + 1 file changed, 1 insertion(+) diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c index c9eeb2ec365d..26e8273bbddd 100644 --- a/drivers/hv/vmbus_drv.c +++ b/drivers/hv/vmbus_drv.c @@ -2192,6 +2192,7 @@ 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 -- 2.25.1
