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

[Igor Mammedov <imammedo@redhat.com>]

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.
 
> Project Context
> ===============
> To refresh your memory on the use case:
> - Objective: Pass `EFI_MEMORY_SP` (Special Purpose Memory) type memory from host to QEMU virtual machine
> - Application: Memory reserved for specific PCI devices (e.g., VFIO-PCI devices)

could you elaborate on this some more /maybe with examples (assume I know nothing about it)?

> - Guest Behavior: The SPM memory should be recognized by the guest OS and claimed by hmem-dax driver
> 
> Complete QEMU Configuration Example:
> -object memory-backend-ram,size=8G,id=m0
> -object memory-backend-file,size=16G,id=m1,mem-path=/dev/dax0.0,prealloc=on,align=1G
> -numa node,nodeid=0,memdev=m0
> -numa node,nodeid=1,memdev=m1,spm=on  # <-- New approach based on your suggestion
> 
> New Patch Implementation
> ========================
> Following your recommendations, I have completely redesigned the implementation:
> 
> Key Changes:
> 1. Removed `hmem` option from `memory-backend-file`
> 2. Added `spm` (special-purpose) option to NUMA node configuration
> 
> I would appreciate your review of the new patch implementation. The design now follows your suggested approach of using NUMA node configuration rather than memory backend options, which should resolve the safety and scope issues we discussed.
> Thank you for your time and valuable guidance on this implementation.
> 
> Please note that I'm located in UTC+8 timezone, so there might be some delay in my responses to your emails due to the time difference. I appreciate your patience and understanding.
> 
> Best regards,
> FangSheng Huang
> 
>