linux-kernel - Re: [PATCH v2] x86/efi: Safely enable unaccepted memory in UEFI

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Message-ID: <20230113222024.rp2erl54vx3grdbd@box.shutemov.name>
Date:   Sat, 14 Jan 2023 01:20:24 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Dionna Glaze <dionnaglaze@...gle.com>
Cc:     linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org,
        x86@...nel.org, jiewen.yao@...el.com, devel@...2.groups.io,
        Ard Biescheuvel <ardb@...nel.org>,
        "Min M. Xu" <min.m.xu@...el.org>,
        Gerd Hoffmann <kraxel@...hat.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        Tom Lendacky <Thomas.Lendacky@....com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v2] x86/efi: Safely enable unaccepted memory in UEFI

On Fri, Jan 13, 2023 at 09:29:26PM +0000, Dionna Glaze wrote:
> This patch depends on Kirill A. Shutemov's series
> 
> [PATCHv8 00/14] mm, x86/cc: Implement support for unaccepted memory
> 
> The UEFI v2.9 specification includes a new memory type to be used in
> environments where the OS must accept memory that is provided from its
> host. Before the introduction of this memory type, all memory was
> accepted eagerly in the firmware. In order for the firmware to safely
> stop accepting memory on the OS's behalf, the OS must affirmatively
> indicate support to the firmware.

I think it is a bad idea.

This approach breaks use case with a bootloader between BIOS and OS.
As the bootloader does ExitBootServices() it has to make the call on
behalf of OS when it has no idea if the OS supports unaccepted.

Note that kexec is such use-case: original kernel has to make a decision
on whether it is okay to leave some memory unaccepted for the new kernel.

And we add this protocol to address very temporary problem: once
unaccepted memory support get upstream it is just a dead weight.

Let's not do this.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov