linux-kernel - Re: [RFC PATCH 1/4] efi/libstub: Decouple memory bitmap from the unaccepted table

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Message-ID: <e66ef586-0e19-4e53-af34-129ed28675c7@amd.com>
Date: Wed, 26 Nov 2025 16:27:19 -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 1/4] efi/libstub: Decouple memory bitmap from the
 unaccepted table

Hi Kiryl,

Thanks for you comments.

On 11/26/25 5:08 AM, Kiryl Shutsemau wrote:
> On Tue, Nov 25, 2025 at 11:57:50AM -0600, Pratik R. Sampat wrote:
>> Memory hotplug in secure environments requires the unaccepted memory
>> bitmap to grow as new memory is added. Currently, the bitmap is
>> implemented as a flexible array member at the end of struct
>> efi_unaccepted_memory, which is reserved by memblock at boot and cannot
>> be resized without reallocating the entire structure.
>>
>> Replace the flexible array member with a pointer.
> 
> Well, it break interoperability between kernel before and after the
> patch. Consider kexec from kernel without the patch to the kernel with
> the patch and then back to older kernel. It is ABI break.
> 
> Is re-allocating the entire structure such a big pain?
> 

We could do that. My concern is that we would then need to protect the
entire table instead of just the bitmap, which may add an additional
overhead?

--
Pratik