Re: [PATCH] numa: add 'spm' option for special purpose memory

[Igor Mammedov <imammedo@redhat.com>]

On Thu, 2 Oct 2025 16:19:00 +0200
David Hildenbrand <david@redhat.com> wrote:

> On 02.10.25 16:11, Igor Mammedov wrote:
> > On Wed, 24 Sep 2025 18:33:23 +0800
> > fanhuang <FangSheng.Huang@amd.com> wrote:
> >   
> >> Hi David,
> >>
> >> I hope this email finds you well. It's been several months since Zhigang last discussion about the Special Purpose Memory (SPM) implementation in QEMU with you, and I wanted to provide some background context before presenting the new patch based on your valuable suggestions.
> >>
> >> Previous Discussion Summary
> >> ===========================
> >> Back in December 2024, we had an extensive discussion regarding my original patch that added the `hmem` option to `memory-backend-file`. During that conversation, you raised several important concerns about the design approach:
> >>
> >> Original Approach (December 2024)
> >> ----------------------------------
> >> - Zhigang's patch: Added `hmem=on` option to `memory-backend-file`
> >> - QEMU cmdline example:
> >>    -object memory-backend-file,size=16G,id=m1,mem-path=/dev/dax0.0,prealloc=on,align=1G,hmem=on
> >>    -numa node,nodeid=1,memdev=m1
> >>
> >> Your Concerns and Suggestions
> >> -----------------------------
> >> You correctly identified some issues with the original approach:
> >> - Configuration Safety: Users could create problematic configurations like:
> >>     -object memory-backend-file,size=16G,id=unused,mem-path=whatever,hmem=on
> >>
> >> - Your Recommendation: You proposed a cleaner approach using NUMA node configuration:
> >>     -numa node,nodeid=1,memdev=m1,spm=on  
> > 
> > that seems to me a bit backwards,
> > aka it's just a particular case where node would have SPM memory only,
> > which (spm) is not a property of numa node, but rather of memory device attached to it.  
> 
> The problem is that boot memory is not modeled as a memory device.

That's historical abomination we currently have.
Question is: does it have to be boot memory, and why?

Also that's why I've asked for use-cases / devices example that would make use of this feature
(VFIO was mentioned here).