linux-kernel - Re: [RFC PATCH 2/4] mm: Add support for unaccepted memory hotplug

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Message-ID: <c8926ced-3ef3-4c11-9d04-00db388887c5@amd.com>
Date: Mon, 1 Dec 2025 11:15:41 -0600
From: "Pratik R. Sampat" <prsampat@....com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>,
 Kiryl Shutsemau <kas@...nel.org>
Cc: linux-mm@...ck.org, linux-coco@...ts.linux.dev,
 linux-efi@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org,
 tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, ardb@...nel.org, akpm@...ux-foundation.org,
 osalvador@...e.de, thomas.lendacky@....com, michael.roth@....com
Subject: Re: [RFC PATCH 2/4] mm: Add support for unaccepted memory hotplug

Hi David,

On 11/28/25 3:34 AM, David Hildenbrand (Red Hat) wrote:
> On 11/27/25 18:40, Kiryl Shutsemau wrote:
>> On Wed, Nov 26, 2025 at 04:27:29PM -0600, Pratik R. Sampat wrote:
>>>
>>>
>>> On 11/26/25 5:12 AM, Kiryl Shutsemau wrote:
>>>> On Tue, Nov 25, 2025 at 11:57:51AM -0600, Pratik R. Sampat wrote:
>>>>> The unaccepted memory structure currently only supports accepting memory
>>>>> present at boot time. The unaccepted table uses a fixed-size bitmap
>>>>> reserved in memblock based on the initial memory layout, preventing
>>>>> dynamic addition of memory ranges after boot. This causes guest
>>>>> termination when memory is hot-added in a secure virtual machine due to
>>>>> accessing pages that have not transitioned to private before use.
>>>>
>>>> How does the hot-pluggable memory look in EFI memory map? I thought
>>>> hot-pluggable ranges suppose to be declared thare. The cleanest solution
>>>> would be to have hot-pluggable and unaccepted indicated in EFI memory,
>>>> so we can size bitmap accordingly upfront.
>>>>
>>>
>>> I'm not quite sure if I fully understand. Do you mean to refer to the
>>> EFI_MEMORY_HOT_PLUGGABLE attribute that is used for cold plugged boot
>>> memory? If so, wouldn't it still be desirable to increase the size of
>>> the bitmap to what was marked as hotpluggable initially?
>>
>> I just don't understand how hotpluggable memory presented in EFI memory
>> map in presence of unaccepted memory. If not-yet-plugged memory marked
>> as unaccepted we can preallocate bitmap upfront and make unaccepted
>> memory transparent wrt hotplug.
>>
>> BTW, isn't virtio-mem a more attractive target to support than HW-style
>> hotplug?
> 
> I would have thought so as well, such that we can just let virtio-mem take care of any acceptance before actually using hotplugged memory (exposing it to the buddy).
> 
> Likely there is desire to support other hypervisors?

That's true. We are certainly thinking about how the RAM discard manager
should look like with multiple states to allow guest_memfd and
virtio-mem to work together.

Since both paths in Linux eventually converge around
add_memory_resource(), based on some light hacking in QEMU I could see
similar hotplug behavior for virtio-mem as well. So I thought I'd get
some feedback on the Linux side of the design since enabling it
for traditional memory seemed like a simpler first step in enabling
hotplug.

Thanks,
--Pratik