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

[Jason Gunthorpe <jgg@ziepe.ca>]

On Tue, May 12, 2026 at 04:08:44PM +0200, Jiri Pirko wrote:
> 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?

Yes, that is what I was thinking

Jason