Re: [PATCH v6 03/20] dma-direct: use DMA_ATTR_CC_SHARED in alloc/free paths

[Jason Gunthorpe]

On Thu, Jun 18, 2026 at 12:39:21PM +1000, Alexey Kardashevskiy wrote:
> 
> 
> On 18/6/26 01:41, Jason Gunthorpe wrote:
> > On Wed, Jun 17, 2026 at 10:50:39AM +1000, Alexey Kardashevskiy wrote:
> > > > @@ -193,16 +193,31 @@ void *dma_direct_alloc(struct device *dev, size_t size,
> > > >    		dma_addr_t *dma_handle, gfp_t gfp, unsigned long attrs)
> > > >    {
> > > >    	bool remap = false, set_uncached = false;
> > > > -	bool mark_mem_decrypt = true;
> > > > +	bool mark_mem_decrypt = false;
> > > >    	struct page *page;
> > > >    	void *ret;
> > > > +	/*
> > > > +	 * DMA_ATTR_CC_SHARED is not a caller-visible dma_alloc_*()
> > > > +	 * attribute. The direct allocator uses it internally after it has
> > > > +	 * decided that the backing pages must be shared/decrypted, so the
> > > > +	 * rest of the allocation path can consistently select DMA addresses,
> > > > +	 * choose compatible pools and restore encryption on free.
> > > 
> > > Why this limit?
> > > 
> > > Context: I am looking for a memory pool for a few shared pages (to
> > > do some guest<->host communication), SWIOTLB seems like the right
> > > fit but swiotlb_alloc() is not exported and
> > > dma_direct_alloc(DMA_ATTR_CC_SHARED) is not allowed.  Thanks,
> > 
> > Then setup your struct device so that the DMA API knows the
> > guest<->host channel requires unecrypted and it will work correctly.
> > 
> > I think this is a reasonable API to use for that, and I was just
> > advocating that hyperv should be using it too.
> > 
> > But it all relies on a properly setup struct device.
> 
> Sounds good but how do I do that in practice? 

I think we haven't got there yet, I understood Dan's plan was to add a
bit in the struct device that signals if the device must be
unencrypted or can support all memory.

Currently the dma api assumes all devices must have unencrypted by
default so it should be fine already, shouldn't it?

> not externally available so I'll have to trick the DMA layer into
> using SWIOTLB (which is still all shared, right?) as I specifically
> want to skip page conversions. Setting low DMA mask won't guarantee
> that the DMA layer won't allocate a page outside of SWIOTLB and
> convert it. Manually do

Why so particular? Any address that satisifies the constraints should
be good enough?

Jason