linux-kernel - Re: [PATCH v7 6/6] x86/efi: Safely enable unaccepted memory in UEFI

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Date:   Wed, 5 Apr 2023 06:00:46 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Ard Biesheuvel <ardb@...nel.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Michael Roth <michael.roth@....com>,
        Joerg Roedel <jroedel@...e.de>,
        Dionna Glaze <dionnaglaze@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Min M. Xu" <min.m.xu@...el.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        Jiewen Yao <jiewen.yao@...el.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v7 6/6] x86/efi: Safely enable unaccepted memory in UEFI

On 4/5/23 00:46, Ard Biesheuvel wrote:
> Once the firmware stops exposing this protocol (and ceases to accept
> memory on the OS's behalf), we can phase it out from the kernel as
> well.

This is a part of the story that I have doubts about.

How and when do you think this phase-out would happen, realistically?

The firmware will need the unaccepted memory protocol support as long as
there are guests around that need it, right?

People like to keep running old kernels for a _long_ time.  Doesn't that
mean _some_ firmware will need to keep doing this dance for a long time?

As long as there is firmware out there in the wild that people want to
run new kernels on, the support needs to stay in mainline.  It can't be
dropped.