On 02.10.25 16:59, Igor Mammedov wrote:
On Thu, 2 Oct 2025 16:19:00 +0200
David Hildenbrand <[email protected]> wrote:
On 02.10.25 16:11, Igor Mammedov wrote:
On Wed, 24 Sep 2025 18:33:23 +0800
fanhuang <[email protected]> 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.
Right, and that's what the memdev= parameter for the node is all about.
Question is: does it have to be boot memory, and why?
I wondered the same in my reply: I'm afraid it cannot be a DIMM/NVDIMM,
these ranges are only described in E820 as "hotplug area".
I think it must be something that's present in the memory map right from
the start, where the OS would identify it as SP and treat it accordingly.
Also that's why I've asked for use-cases / devices example that would make use
of this feature
(VFIO was mentioned here).
Yes, good point.
--
Cheers
David / dhildenb
