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

[Jiri Pirko <jiri@resnulli.us>]

Sun, May 17, 2026 at 01:50:06PM +0200, leon@kernel.org wrote:
>On Fri, May 15, 2026 at 08:13:17AM +0200, Jiri Pirko wrote:
>> Thu, May 14, 2026 at 06:25:06PM +0200, leon@kernel.org wrote:
>> >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.
>> 
>> This patch doesn't do that. The patch description only describes the
>> current situation and how the patch changes the behaviour.
>
>From previous patch:
>+       if (dma_device &&
>+           cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) &&
>+           is_swiotlb_force_bounce(dma_device))
>+               device->cc_dma_bounce = 1;
>
>And this patch reads cc_dma_bounce.

Sure, I got that remark from the reply to the previous patch :) Already
removed. Will send next v soon.

Thanks!