RE: [PATCH 1/1] Drivers: hv: vmbus: Set DMA coherent mask for VMBus devices

[Michael Kelley]

From: Michael Kelley <mhklkml@xxxxxxxxxxxx> Sent: Sunday, June 7, 2026 7:06 PM
> 
> 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@xxxxxxxxxxx>

Gentle ping:  Anyone able to review this patch? There's a
Sashiko comment, but it's for an issue in an unrelated error path,
so I'm not planning to respin this patch for that comment.

Michael

> ---
> 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
>