Re: [PATCH v4 0/1] numa: add 'spm' option for Specific Purpose Memory

[Igor Mammedov <imammedo@redhat.com>]

On Tue, 9 Dec 2025 17:38:40 +0800
fanhuang <FangSheng.Huang@amd.com> wrote:

> Hi all,
> 
> This is v4 of the SPM (Specific Purpose Memory) patch. Thank you Jonathan
> for the detailed review.
> 
> Changes in v4 (addressing Jonathan's feedback):
> - Added architecture check: spm=on now reports error on non-x86 machines
> - Simplified return logic in e820_update_entry_type() (return true/false directly)
> - Changed 4GB boundary spanning from warn_report to error_report + exit
> - Updated QAPI documentation to be architecture-agnostic (removed E820 reference)
> - Removed unnecessary comments
> 
> Use case:
> This feature allows passing EFI_MEMORY_SP (Specific Purpose Memory) from
> host to guest VM, useful for memory reserved for specific PCI devices
> (e.g., GPU memory via VFIO-PCI). The SPM memory appears as soft reserved
> to the guest and is managed by device drivers rather than the OS memory
> allocator.
> 
> Example usage:
>   -object memory-backend-ram,size=8G,id=m0
>   -object memory-backend-file,size=8G,id=m1,mem-path=/dev/dax0.0
>   -numa node,nodeid=0,memdev=m0
>   -numa node,nodeid=1,memdev=m1,spm=on

I'm still not fond of 'spm' toggle on numa node itself (even though on AMD hadware sunch memory has 1:1 mapping)
without device model in between.

Can we try following instead:
  * add 'spm' property to DIMM device and disable hotplug on it in such case
  * make E820 enumerate spm/not hotpluggble marked DIMMs.

That will let us later to have mixed memory on the node if such need arises without
breaking QEMU CLI.

> Please review. Thanks!
> 
> Best regards,
> Jerry Huang
>