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

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Message-ID: <20230116231711.cudsnxvnfg6aef3w@box.shutemov.name>
Date:   Tue, 17 Jan 2023 02:17:11 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Dionna Amalie Glaze <dionnaglaze@...gle.com>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        Gerd Hoffmann <kraxel@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org,
        x86@...nel.org, jiewen.yao@...el.com, devel@...2.groups.io,
        "Min M. Xu" <min.m.xu@...el.org>,
        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 Mon, Jan 16, 2023 at 11:43:15AM -0800, Dionna Amalie Glaze wrote:
> > > I still don't understand why we need to support every imaginable
> > > combination of firmware, bootloader and OS. Unaccepted memory only
> > > exists on a special kind of virtual machine, which provides very
> > > little added value unless you opt into the security and attestation
> > > features, which are all heavily based on firmware protocols. So why
> > > should care about a EFI-aware bootloader calling ExitBootServices()
> > > and subsequently doing a legacy boot of Linux on such systems?
> >
> > Why break what works? Some users want it.
> >
> 
> The users that want legacy boot features will not be broken,

Why do you call boot with a bootloader a legacy feature?

> they'll only get a safe view of the memory map. I don't think it's right
> to choose unsafe behavior for a legacy setup.

Present memory map with unaccepted memory to OS that doesn't about it is
perfectly safe. This portion of the memory will be ignored. It is "feature
not [yet] implemented" case.

> > This patch adds complexity, breaks what works and the only upside will
> > turn into a dead weight soon.
> >
> > There's alternative to add option to instruct firmware to accept all
> > memory from VMM side. It will serve legacy OS that doesn't know about
> > unaccepted memory and it is also can be use by latency-sensitive users
> > later on (analog of qemu -mem-prealloc).
> >
> 
> This means that users of a distro that has not enabled unaccepted
> memory support cannot simply start a VM with the usual command, but
> instead have to know a baroque extra flag to get access to all the
> memory that they configured the machine (and for a CSP customer, paid
> for). That's not a good experience.

New features require enabling. It is not something new.

> With GCE at least, you can't (shouldn't) associate the boot feature
> flag with a disk image because disks are mutable. If a customer
> upgrades their kernel after initially starting their VM, they can't
> remove the flag due to the way image annotations work.

I guess a new VM has to be created, right? Doesn't sound like a big deal
to me.

The old will not break with upgraded kernel. Just not get benefit of the
feature.

> All of this headache goes away by adopting a small patch to the kernel
> that calls a 0-ary protocol interface and keeping safe acceptance
> behavior in the firmware. I think Gerd is right here that we should
> treat it as a transition feature that we can remove later.

Removing a feature is harder than adding one. How do you define that
"later" has come?

Anyway, I think we walk in a circle. I consider it a misfeature. If you
want still go this path, please add my

Nacked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov