Re: [PATCH v7 16/22] dma-direct: make dma_direct_map_phys() honor DMA_ATTR_CC_SHARED

[Jason Gunthorpe <jgg@ziepe.ca>]

On Thu, Jul 09, 2026 at 12:13:19PM +0100, Catalin Marinas wrote:
> > > For AMD/SME, on host with memory encryption we now end up setting the C
> > > bit for DMA_ATTR_MMIO. This is fine for RAM but not sure whether
> > > some other MMIO bus understands this attribute. Maybe we should stick to
> > > something like __phys_to_dma() for the !CC_SHARED && MMIO path. Or,
> > > since this is not universally defined, just use the old dma_addr = phys
> > > if MMIO and ignore any unlikely DMA offsets.
> > >
> > 
> > Considering for AMD/SME system an unencrypted dma addr is one without C
> > bit, will this be good?
> > 
> > 	/*
> > 	 * For host memory encryption and device requiring unencrypted DMA,
> > 	 * MMIO memory is treated as shared by default.
> > 	 */
> > 	if (attrs & DMA_ATTR_MMIO) {
> > 		if (cc_platform_has(CC_ATTR_HOST_MEM_ENCRYPT) || force_dma_unencrypted(dev))
> > 			attrs |= DMA_ATTR_CC_SHARED;
> > 	}
> 
> Yes, I think it does the trick, preserves the current semantics for AMD.
> I guess you could use a single 'if' for all checks (up to you).

Please don't change it, MMIO P2P is broken on CC systems today and it
should stay broken. Passing DMA_ATTR_MMIO with DMA_ATTR_CC_SHARED is
an error that we need to correct in the drivers not make work in the
core code.

Jason