Re: [PATCH v2 1/4] kernel/resource: Introduce managed SOFT RESERVED resources

[Fan Ni <nifan.cxl@gmail.com>]

On Tue, Jan 21, 2025 at 12:57:19PM -0600, Fontenot, Nathan wrote:
> 
> 
> On 1/21/2025 2:19 AM, David Hildenbrand wrote:
> > On 16.01.25 18:42, Nathan Fontenot wrote:
> > 
> > Hi,
> > 
> >> Introduce the ability to manage SOFT RESERVED kernel resources prior to
> >> these resources being  placed in the iomem resource tree. This provides
> >> the ability for drivers to update SOFT RESERVED resources that intersect
> >> with their memory resources.
> >>
> >> During boot, any resources marked as IORES_DESC_SOFT_RESERVED are placed
> >> on the soft reserve resource tree. Once boot completes all resources
> >> are placed on the iomem resource tree. This behavior is gated by a new
> >> kernel option CONFIG_SOFT_RESERVED_MANAGED.
> >>
> > 
> > I'm missing a bit of context here.
> > 
> > Why can't we flag these regions in the existing iomem tree, where they can be fixed up (even after boot?)?
> > 
> > Especially, what about deferred driver loading after boot? Why is that not a concern or why can we reliably handle everything "during boot" ?
> 
> That's a good question and one I should have addressed.
> 
> The goal is to prevent the dax driver from creating dax devices for soft reserve
> resources prior to the soft reserve resources being updated for any intersecting
> cxl regions.

Not an export. Can you explain a little more here?
What is the problem if we only flag the resources as "soft
reserved" in the iomem tree without creating a separate tree, and
process the "soft reserved" resources only when needed? 

Fan
> 
> During boot the dax hmem driver walks the iomem tree to save off a copy of all
> soft reserve resources. The dax driver then later walks this copy to create
> dax devices for the soft reserve regions. This occurs before the cxl drivers
> load, create cxl regions, and update any intersecting soft reserve resources.
> 
> To prevent this the soft reserves are set aside on a separate list during boot
> so that they can be updated (if needed) and later added to the iomem resource tree.
> The dax driver is then notified of any soft reserves added to the iomem tree
> so that it may consume them.
> 
> Hopefully that answers your question. I'll include this in the next version.
> 
> -Nathan
> 
> > 
> >> As part of this update two new interfaces are added for management of
> >> the SOFT RESERVED resources. The release_srmem_region_adjustable()
> >> routine allows for removing pieces of SOFT RESERVED resources. The
> >> the merge_srmem_resources() allows drivers to merge any remaining SOFT
> >> RESERVED resources into the iomem resource tree once updatea are complete.
> >>
> >> Signed-off-by: Nathan Fontenot <nathan.fontenot@amd.com>
> > 
>