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

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Message-ID: <ac627386-eeb5-46d4-9d4b-dfb902280370@amd.com>
Date: Mon, 1 Dec 2025 11:58:02 -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 12/1/25 11:48 AM, Kiryl Shutsemau wrote:
> On Mon, Dec 01, 2025 at 11:15:13AM -0600, Pratik R. Sampat wrote:
>>
>>
>> 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?
> 
> 4k per 64GiB of physical address space should be low enough to ignore, no?
> 
> We can look into optimizing it out when it is an actual, not imaginary
> problem.
> 

Sure, that's fair!

--Pratik