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

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Date:   Tue, 4 Apr 2023 21:09:17 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        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 Tue, Apr 04, 2023 at 10:57:52AM -0700, Dave Hansen wrote:
> 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?

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.

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.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov