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

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Message-ID: <maleev6ofzlnhi3rmqjawlllxkda4mrgwmb6msz5gz77klfrxl@adpqtakqrxrj>
Date: Mon, 1 Dec 2025 17:48:39 +0000
From: Kiryl Shutsemau <kas@...nel.org>
To: "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, 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 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.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov