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

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Message-ID: <96513ddd-ee87-5fae-cb5c-79d0120fd326@intel.com>
Date:   Wed, 5 Apr 2023 14:22:47 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Tom Lendacky <thomas.lendacky@....com>,
        "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Ard Biesheuvel <ardb@...nel.org>, 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 13:11, Tom Lendacky wrote:
>>> The thing that worries me is the "Near future firmware" where someone
>>> runs a ~6.4 kernel and has a fast boot experience.  They upgrade to a
>>> newer, "dropped protocol" kernel and their boot gets slower.
> 
> Right, so that is what begs the question of when to actually drop the
> call. Or does it really need to be dropped? It's a small patch to
> execute a boot services call, I guess I don't see the big deal of it
> being there.
> If the firmware still has the protocol, the call is made, if it doesn't,
> its not. In the overall support for unaccepted memory, this seems to be
> a very minor piece.

I honestly don't think it's a big deal either, at least on the kernel
side.  Maybe it's a bigger deal to the firmware folks on their side.

So, the corrected table looks something like this:

              |                   Kernel                   |
              |                                            |
              | Unenlightened | Enlightened | Dropped UEFI |
Firmware      |     ~5.19??   |    ~6.4??   | protocol     |
              |---------------+-------------+--------------|
Deployed      |   Slow boot   |  Slow boot  |  Slow boot   |
Near future   |   Slow boot   |  Fast boot  |  Slow boot   |
Far future    |  2GB limited  |  Fast Boot  |  Fast boot   |


But, honestly, I don't see much benefit to the "dropped UEFI protocol".
It adds complexity and will represent a regression either in boot
speeds, or in unenlightened kernels losing RAM when moving to newer
firmware.  Neither of those is great.

Looking at this _purely_ from the kernel perspective, I think I'd prefer
this situation:

          |            Kernel           |
          |                             |
          | Unenlightened | Enlightened |
Firmware  |     ~5.19??   |    ~6.4??   |
          |---------------+-------------+
Deployed  |   Slow boot   |  Slow boot  |
Future    |   Slow boot   |  Fast boot  |

and not have future firmware drop support for the handshake protocol.
That way there are no potential regressions.

Is there a compelling reason on the firmware side to drop the
ExitBootServices() protocol that I'm missing?