Re: [RFCv2 PATCH 0/6] Support memory hotplug/unplug for TDX CoCo guests

["Pratik R. Sampat" <prsampat@amd.com>]

On 8/27/26 4:18 AM, Duan, Zhenzhong wrote:
> 
> 
>> -----Original Message-----
>> From: David Hildenbrand (Arm) <david@kernel.org>
>> Subject: Re: [RFCv2 PATCH 0/6] Support memory hotplug/unplug for TDX CoCo
>> guests
>>
>> On 6/23/26 12:17, Zhenzhong Duan wrote:
>>> This RFCv2 series implements comprehensive support for virtio-mem and ACPI
>>> DIMM memory hotplug/unplug in Intel TDX confidential computing guests.
>>> It explores the start-private memory approach utilizing the native
>>> TDG.MEM.PAGE.RELEASE API.
>>>
>>> We are seeking feedback from Kiryl on the CoCo guest implementation, MM
>>> experts on DIMM & virio-mem memory hotplug integration and broader
>>> virtio/CoCo community input on the overall approach. We are not seeking
>>> x86 maintainer review at this stage.
>>>
>>> == Changes from RFC v1 ==
>>>
>>> - Eliminated callback infrastructure: Dropped plug callback and replaced
>>>   unplug callback with platform-level unaccept function into core MM
>>>   hotplug and virtio-mem subsystems.
>>> - Added comprehensive bitmap tracking: Introduced a "plugged" bitmap
>>>   alongside the unaccepted bitmap to track populated hotplug memory
>>>   states to support load_unaligned_zeropad().
>>> - Enhanced SRAT parsing: Extended the EFI stub to parse ACPI SRAT tables
>>>   early, ensuring hotpluggable ranges are tracked from initial boot.
>>>
>>> For more introduction about the background or other efforts in community,
>>> please check the RFCv1 cover letter [1].
>>>
>>> == Technical Approach ==
>>>
>>> - Early SRAT Integration: A lightweight EFI stub parser scans ACPI SRAT
>>>   tables to identify hotpluggable ranges and adjust bitmap boundaries
>>>   early, avoiding the overhead of the full ACPI subsystem.
>>> - Comprehensive Bitmap Tracking: Introduces a "plugged" bitmap right
>>>   after the unaccepted bitmap. Both static and hotplugged memory are
>>>   tracked, allowing the guest to map which ranges are populated by the
>>>   VMM. This prevents acceptance beyond plugged memory boundaries due to
>>>   load_unaligned_zeropad() operations.
>>> - Platform Extensibility: Exposes generic CoCo memory interfaces. Other
>>>   confidential platforms (like AMD SEV-SNP) can easily adopt this by
>>>   hooking their specific mechanisms into arch_unaccept_memory().
>>> - Hotplug & Guest Control: Integrates platform-level unaccept logic
>>>   into ACPI hotplug and virtio-mem handlers. Uses TDG.MEM.PAGE.RELEASE
>>>   for TDX to explicitly set memory to the "unaccepted" state during
>>>   unplug, removing host hole-punching dependencies.
>>> - Kexec Handover: Leverages existing EFI mechanisms to seamlessly hand
>>>   over both the extended unaccepted bitmap and the new plugged bitmap
>>>   across kexec boundaries.
>>>
>>> == Testing ==
>>>
>>> - dimm and virtio-mem memory hotplug/unplug
>>> - lazy and eager accept
>>> - kexec/kdump with hotplugged memory
>>>
>>> This is tested with Marc-André Lureau's newest qemu series [2]
>>
>> What's the status of this?
> 
> Marc's QEMU series is merged.
> For this series, following feedback from Kirill and Pratik, the preferred approach
> is updating the UEFI spec for hotplug memory ranges rather than parsing SRAT
> at the EFI stage. Pratik is already pushing this forward, I am currently waiting on
> his RFCs. If he hasn't taken over the entire implementation, I can rebase my
> remaining patches on top of his work.
> 
> Hi Pratik, have you sent your UEFI RFC out yet? Just wanted to make sure
> I didn't miss your thread.
> 

Yes, the RFC has been out for a bit: https://github.com/tianocore/edk2/pull/12811

The general feedback I've received is that folks are wary of adding yet another
attribute to support hotplug memory (beyond EFI_MEMORY_SP and
EFI_MEMORY_HOT_PLUGGABLE), and that they're waiting for more community input
before moving forward.

In hindsight, I should probably have pinged here as well since folks on this
thread are more adept to review.

Thanks,
--Pratik