[PATCH v9 10/24] mm/hmm: let users to tag specific PFN with DMA mapped bit

[Jason Gunthorpe]

On Wed, Apr 23, 2025 at 08:54:05PM +0300, Mika Penttilä wrote:
> > @@ -36,6 +38,13 @@ enum hmm_pfn_flags {
> >  	HMM_PFN_VALID = 1UL << (BITS_PER_LONG - 1),
> >  	HMM_PFN_WRITE = 1UL << (BITS_PER_LONG - 2),
> >  	HMM_PFN_ERROR = 1UL << (BITS_PER_LONG - 3),
> > +
> > +	/*
> > +	 * Sticky flags, carried from input to output,
> > +	 * don't forget to update HMM_PFN_INOUT_FLAGS
> > +	 */
> > +	HMM_PFN_DMA_MAPPED = 1UL << (BITS_PER_LONG - 7),
> > +
> 
> How is this playing together with the mapped order usage?

Order shift starts at bit 8, DMA_MAPPED is at bit 7

The pfn array is linear and simply indexed. The order is intended for
page table like HW to be able to build larger entries from the hmm
data without having to scan for contiguity.

Even if order is present the entry is still replicated across all the
pfns that are inside the order.

At least this series should replicate the dma_mapped flag as well as
it doesn't pay attention to order.

I suspect a page table implementation may need to make some small
changes. Indeed with guarenteed contiguous IOVA there may be a
significant optimization available to have the HW page table cover all
the contiguous present pages in the iommu, which would be a higher
order than the pages themselves. However this would require being able
to punch non-present holes into contiguous mappings...

Jason