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

Browse source

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>

...

This commit is contained in:

Michael Kelley 2026-06-07 19:06:16 -07:00 • committed by Wei Liu

parent d6f0248f04

commit ab96bc97cd

1 changed files with 1 additions and 0 deletions

Show all changes Ignore whitespace when comparing lines Ignore changes in amount of whitespace Ignore changes in whitespace at EOL

Download patch file Download diff file Expand all files Collapse all files

1

drivers/hv/vmbus_drv.c

View file