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

[Jiri Pirko <jiri@resnulli.us>]

Tue, May 12, 2026 at 04:05:10PM CEST, jgg@ziepe.ca wrote:
>On Tue, May 12, 2026 at 04:03:07PM +0200, Jiri Pirko wrote:
>> >> @@ -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.
>> 
>> What do you suggest as alternative? We need to somehow tell the user
>> what is the situation.
>
>For now CC_ATTR_GUEST_MEM_ENCRYPT is likely sufficient.
>
>Later we should be able to detect if the device is in T=1 mode
>directly.

Okay, so we assume for now that every device is T=0 (which I believe is
the reality). Once T=1 device appears, it changes this "if statement".
Do I understand that correctly?