[PATCH 1/9] block: don't merge different kinds of P2P transfers in a single bio

[Christoph Hellwig]

On Tue, Jun 10, 2025 at 09:37:30AM -0600, Keith Busch wrote:
> On Tue, Jun 10, 2025 at 07:06:39AM +0200, Christoph Hellwig wrote:
> > To get out of the DMA mapping helpers having to check every segment for
> > it's P2P status, ensure that bios either contain P2P transfers or non-P2P
> > transfers, and that a P2P bio only contains ranges from a single device.
> 
> I may be out of the loop here. Is this an optimization to make something
> easier for the DMA layer?

Yes.  P2P that is based on a bus address (i.e. using a switch) uses
a completely different way to DMA MAP than the normal IOMMU or
direct mapping.  So the optimization of collapsing all host physical
addresses into an iova can't work once it is present.

> I don't think there's any fundamental reason
> why devices like nvme couldn't handle a command that uses memory mixed
> among multiple devices and/or host memory, at least.

Sure, devices don't even see if an IOVA is P2P or not, this is all
host side.