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

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Message-ID: <v7ocifp2adtdptkbzjmvrhqgjjbzug3a5dqspw54kmfmpauwdz@fuwlvrnjhpcm>
Date: Thu, 27 Nov 2025 17:29:39 +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 1/4] efi/libstub: Decouple memory bitmap from the
 unaccepted table

On Wed, Nov 26, 2025 at 04:27:19PM -0600, Pratik R. Sampat wrote:
> 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?

What additional overhead? The main contention is going to be on binmap
anyway.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov