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

[Leon Romanovsky <leon@kernel.org>]

On Tue, May 12, 2026 at 04:04:13PM +0200, Jiri Pirko wrote:
> Tue, May 12, 2026 at 03:05:15PM CEST, leon@kernel.org wrote:
> >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".
> 
> I'm not sure I follow. What's the issue you see?

SWIOTLB is the layer below DMA API, RDMA is the layer above DMA API.
You shouldn't call to SWIOTLB functions in RDMA code.

Thanks

> 
> 
> 
> 
> >
> >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
> >>