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

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Date:   Tue, 4 Apr 2023 10:57:52 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Tom Lendacky <thomas.lendacky@....com>
Cc:     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>,
        Ard Biescheuvel <ardb@...nel.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 4/4/23 10:45, Kirill A. Shutemov wrote:
> I still think it is a bad idea.
> 
> As I asked before, please include my
> 
> Nacked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> 
> into the patch.

I was pretty opposed to this when I first saw it too.  But, Tom and
company have worn down my opposition a bit.

The fact is that we have upstream kernels out there with SEV-SNP support
that don't know anything about unaccepted memory.  They're either
relegated to using the pre-accepted memory (4GB??) or _some_ entity
needs to accept the memory.  That entity obviously can't be the kernel
unless we backport unaccepted memory support.

This both lets the BIOS be the page-accepting entity _and_ allows the
entity to delegate that to the kernel when it needs to.

As much as I want to nak this and pretend that that those existing
kernel's don't exist, my powers of self-delusion do have their limits.

If our AMD friends don't do this, what is their alternative?