Re: [RFC PATCH 00/28] Removing struct page from P2PDMA

From: Jason Gunthorpe
Date: Mon Jun 24 2019 - 09:55:56 EST

On Mon, Jun 24, 2019 at 03:50:24PM +0200, Christoph Hellwig wrote:
> On Mon, Jun 24, 2019 at 10:46:41AM -0300, Jason Gunthorpe wrote:
> > BTW, it is not just offset right? It is possible that the IOMMU can
> > generate unique dma_addr_t values for each device?? Simple offset is
> > just something we saw in certain embedded cases, IIRC.
> Yes, it could. If we are trying to do P2P between two devices on
> different root ports and with the IOMMU enabled we'll generate
> a new bus address for the BAR on the other side dynamically everytime
> we map.

Even with the same root port if ACS is turned on could behave like this.

It is only a very narrow case where you can take shortcuts with
dma_addr_t, and I don't think shortcuts like are are appropriate for
the mainline kernel..