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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Tue, 9 Aug 2022 13:36:00 +0200
Message-ID: <CAMj1kXE67H_cgYbufUxQ7HXg929dFopWH+cKX5ijAdePP8Zt-g@mail.gmail.com>
Subject: Re: [PATCHv7 00/14] mm, x86/cc: Implement support for unaccepted memory
To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc:     Dave Hansen <dave.hansen@intel.com>, Marc Orr <marcorr@google.com>,
        Borislav Petkov <bp@alien8.de>,
        Dionna Amalie Glaze <dionnaglaze@google.com>,
        Peter Gonda <pgonda@google.com>,
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        Sean Christopherson <seanjc@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Joerg Roedel <jroedel@suse.de>,
        Andi Kleen <ak@linux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
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On Tue, 9 Aug 2022 at 13:11, Kirill A. Shutemov
<kirill.shutemov@linux.intel.com> wrote:
>
> On Sat, Jul 23, 2022 at 01:14:07PM +0200, Ard Biesheuvel wrote:
> > On Thu, 21 Jul 2022 at 19:13, Dave Hansen <dave.hansen@intel.com> wrote:
> > >
> > > On 7/19/22 17:26, Marc Orr wrote:
> > > > - Dave's suggestion to "2. Boot some intermediate thing like a
> > > > bootloader that does acceptance ..." is pretty clever! So if upstream
> > > > thinks this FW-kernel negotiation is not a good direction, maybe we
> > > > (Google) can pursue this idea to avoid introducing yet another tag on
> > > > our images.
> > >
> > > I'm obviously speaking only for myself here and not for "upstream" as a
> > > whole, but I clearly don't like the FW/kernel negotiation thing.  It's a
> > > permanent pain in our necks to solve a very temporary problem.
> >
> > EFI is basically our existing embodiment of this fw/kernel negotiation
> > thing, and iff we need it, I have no objection to using it for this
> > purpose, i.e., to allow the firmware to infer whether or not it should
> > accept all available memory on behalf of the OS before exiting boot
> > services. But if we don't need this, even better.
>
> FW/kernel negotiation does not work if there's a boot loader in the middle
> that does ExitBootServices(). By the time kernel can announce if it
> supports unaccepted memory there's nobody to announce to.
>

Why would you want to support such bootloaders for TDX anyway? TDX
heavily relies on measured boot abstractions and other things that are
heavily tied to firmware.