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

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Message-ID: <27c06cf2-7500-4875-bd22-f55571fb85f9@kernel.org>
Date: Fri, 28 Nov 2025 10:34:04 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Kiryl Shutsemau <kas@...nel.org>, "Pratik R. Sampat" <prsampat@....com>
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

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?

-- 
Cheers

David