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

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Message-ID: <66ylzwknm4ftd6utn3nqr63jmhl2ccvcdvyi5fechfnvmfxivu@37pckhjixayh>
Date: Wed, 26 Nov 2025 11:12:13 +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 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.

> Extend the unaccepted memory framework to handle hotplugged memory by
> dynamically managing the unaccepted bitmap. Allocate a new bitmap when
> hotplugged ranges exceed the reserved bitmap capacity and switch to
> kernel-managed allocation.
> 
> Hotplugged memory also follows the same acceptance policy using the
> accept_memory=[eager|lazy] kernel parameter to accept memory either
> up-front when added or before first use.
> 
> Signed-off-by: Pratik R. Sampat <prsampat@....com>
> ---
>  arch/x86/boot/compressed/efi.h                |  1 +
>  .../firmware/efi/libstub/unaccepted_memory.c  |  1 +
>  drivers/firmware/efi/unaccepted_memory.c      | 83 +++++++++++++++++++
>  include/linux/efi.h                           |  1 +
>  include/linux/mm.h                            | 11 +++
>  mm/memory_hotplug.c                           |  7 ++
>  mm/page_alloc.c                               |  2 +
>  7 files changed, 106 insertions(+)
> 
> diff --git a/arch/x86/boot/compressed/efi.h b/arch/x86/boot/compressed/efi.h
> index 4f7027f33def..a220a1966cae 100644
> --- a/arch/x86/boot/compressed/efi.h
> +++ b/arch/x86/boot/compressed/efi.h
> @@ -102,6 +102,7 @@ struct efi_unaccepted_memory {
>  	u32 unit_size;
>  	u64 phys_base;
>  	u64 size;
> +	bool mem_reserved;
>  	unsigned long *bitmap;
>  };
>  

Again, this is ABI break for kexec.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov