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

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Message-ID: <f60d968e-6be3-4bad-8bf4-8500ad039817@amd.com>
Date: Mon, 1 Dec 2025 11:15:13 -0600
From: "Pratik R. Sampat" <prsampat@....com>
To: 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,
 david@...hat.com, 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 11:40 AM, 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.

If memory that hasn't been plugged yet never gets plugged in or is only
partially plugged in, wouldn't we be wasting space by preallocating
the bitmap upfront? Or would that not be a concern in favor of
transparency?

--Pratik