Re: [PATCH rdma-next v2 2/2] RDMA/umem: block plain userspace memory registration under CoCo bounce

[Leon Romanovsky <leon@kernel.org>]

On Wed, May 06, 2026 at 01:14:47PM +0200, Jiri Pirko wrote:
> From: Jiri Pirko <jiri@nvidia.com>
> 
> When a device requires DMA bounce buffering inside a Confidential
> Computing guest, __ib_umem_get_va() cannot work. The DMA mapping layer
> redirects all mappings through swiotlb bounce buffers, so the device
> receives DMA addresses pointing to bounce buffer memory rather than
> the user's pages. 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.
> 
> The registration would already fail later on, since the umem mapping
> is requested with DMA_ATTR_REQUIRE_COHERENT and gets rejected under
> is_swiotlb_force_bounce() with -EIO. Fail early with -EOPNOTSUPP
> instead, so the user gets a specific error code to react to.

DMA_ATTR_REQUIRE_COHERENT was our answer to "layering violation claim".

Thanks

> 
> Signed-off-by: Jiri Pirko <jiri@nvidia.com>
> ---
> v1->v2:
> - updated patch description with mention of DMA_ATTR_REQUIRE_COHERENT
> ---
>  drivers/infiniband/core/umem.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/infiniband/core/umem.c b/drivers/infiniband/core/umem.c
> index 611d693eb9a2..b1877b83b021 100644
> --- a/drivers/infiniband/core/umem.c
> +++ b/drivers/infiniband/core/umem.c
> @@ -167,6 +167,9 @@ static struct ib_umem *__ib_umem_get_va(struct ib_device *device,
>  	int pinned, ret;
>  	unsigned int gup_flags = FOLL_LONGTERM;
>  
> +	if (device->cc_dma_bounce)
> +		return ERR_PTR(-EOPNOTSUPP);
> +
>  	/*
>  	 * If the combination of the addr and size requested for this memory
>  	 * region causes an integer overflow, return error.
> -- 
> 2.53.0
>