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Date:   Tue, 4 Apr 2023 12:27:43 -0700
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Subject: Re: [PATCH v7 6/6] x86/efi: Safely enable unaccepted memory in UEFI
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To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     Tom Lendacky <thomas.lendacky@amd.com>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Michael Roth <michael.roth@amd.com>,
        Joerg Roedel <jroedel@suse.de>,
        Dionna Glaze <dionnaglaze@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ard Biescheuvel <ardb@kernel.org>,
        "Min M. Xu" <min.m.xu@intel.com>,
        Gerd Hoffmann <kraxel@redhat.com>,
        James Bottomley <jejb@linux.ibm.com>,
        Jiewen Yao <jiewen.yao@intel.com>,
        Erdem Aktas <erdemaktas@google.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
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From:   Dave Hansen <dave.hansen@intel.com>
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On 4/4/23 11:09, Kirill A. Shutemov wrote:
>> If our AMD friends don't do this, what is their alternative?
> The alternative is coordination on the host side: VMM can load a BIOS that
> pre-accepts all memory if the kernel is older.
> 
> I know that it is not convenient for VMM, but it is technically possible.

Yeah, either a specific BIOS or a knob to tell the BIOS what it has to
do.  But, either way, that requires coordination between the BIOS (or
BIOS configuration) and the specific guest.  I can see why that's
unpalatable.

> Introduce an ABI with an expiration date is much more ugly. And nobody
> will care about the expiration date, until you will try to remove it.

Yeah, the only real expiration date for an ABI is "never".  I don't
believe for a second that we'll ever be able to remove the interface.

Either way, I'd love to hear more from folks about why a BIOS-side
option (configuration or otherwise) is not a good option.  I know we've
discussed this in a few mail threads, but it would be even better to get
it into the cover letter or documentation.