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

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Message-ID: <20230116105648.63hsxnmj2juwudmu@sirius.home.kraxel.org>
Date:   Mon, 16 Jan 2023 11:56:48 +0100
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Dionna Glaze <dionnaglaze@...gle.com>,
        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>,
        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 Sat, Jan 14, 2023 at 01:20:24AM +0300, Kirill A. Shutemov wrote:
> 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.

Nothing breaks, it'll error on the safe side.  If the protocol callback
is not called the firmware will simply accept all memory.  The guest OS
will only see unaccepted memory if it explicitly asked for it (assuming
the firmware wants know to support both cases, of course the firmware
could also enforce the one or the other and just not offer the
protocol).

> 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.

Not sure what you are trying to tell.  The kexec case doesn't go
through the efi stub anyway.

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

Maybe, maybe not.  unaccepted memory support has a Kconfig switch after
all.  If we figure in 3-5 years that all distros have enabled it anyway
we can drop it again.  For the transition period it will surely be
useful.

take care,
  Gerd