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

[Wei Liu]

On Thu, Jul 16, 2026 at 05:57:49PM +0000, Michael Kelley wrote:
> 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.

Applied. Thanks.