Re: [PATCH rdma-next v2 1/2] RDMA/uverbs: expose CoCo DMA bounce requirement to userspace

[Leon Romanovsky <leon@kernel.org>]

On Wed, May 06, 2026 at 01:14:46PM +0200, Jiri Pirko wrote:
> From: Jiri Pirko <jiri@nvidia.com>
> 
> In CoCo guests, device DMA to regular userspace memory does not work
> because the DMA mapping layer redirects all mappings through swiotlb
> bounce buffers. Since RDMA devices access registered memory directly
> without CPU involvement, there is no opportunity for swiotlb to
> synchronize between the bounce buffer and the original pages.
> 
> Expose this condition to userspace as IB_UVERBS_DEVICE_CC_DMA_BOUNCE
> in device_cap_flags_exi.
> 
> Signed-off-by: Jiri Pirko <jiri@nvidia.com>
> ---
>  drivers/infiniband/core/device.c     | 6 ++++++
>  drivers/infiniband/core/uverbs_cmd.c | 2 ++
>  include/rdma/ib_verbs.h              | 3 +++
>  include/uapi/rdma/ib_user_verbs.h    | 2 ++
>  4 files changed, 13 insertions(+)
> 
> diff --git a/drivers/infiniband/core/device.c b/drivers/infiniband/core/device.c
> index b89efaaa81ec..ad3da92c9318 100644
> --- a/drivers/infiniband/core/device.c
> +++ b/drivers/infiniband/core/device.c
> @@ -42,6 +42,8 @@
>  #include <linux/security.h>
>  #include <linux/notifier.h>
>  #include <linux/hashtable.h>
> +#include <linux/cc_platform.h>
> +#include <linux/swiotlb.h>
>  #include <rdma/rdma_netlink.h>
>  #include <rdma/ib_addr.h>
>  #include <rdma/ib_cache.h>
> @@ -1419,6 +1421,10 @@ int ib_register_device(struct ib_device *device, const char *name,
>  	 */
>  	WARN_ON(dma_device && !dma_device->dma_parms);
>  	device->dma_device = dma_device;
> +	if (dma_device &&
> +	    cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) &&
> +	    is_swiotlb_force_bounce(dma_device))

It is the wrong place. When I worked on my DMA series, I tried something
similar (a call into SWIOTLB) to notify users that RDMA would not work.

The general feedback was that this is a layering violation, and that any
knowledge of SWIOTLB (and its API) should not leak out of the DMA API.

You shouldn't call to is_swiotlb_force_bounce() here.

Thanks