Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Mon, 18 Jul 2022 20:21:59 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Peter Gonda <pgonda@google.com>, Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>,
        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>,
        Vlastimil Babka <vbabka@suse.cz>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Ingo Molnar <mingo@redhat.com>,
        Varad Gautam <varad.gautam@suse.com>,
        Dario Faggioli <dfaggioli@suse.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Mike Rapoport <rppt@kernel.org>,
        David Hildenbrand <david@redhat.com>,
        Marcelo Cerri <marcelo.cerri@canonical.com>,
        tim.gardner@canonical.com,
        Khalid ElMously <khalid.elmously@canonical.com>,
        philip.cox@canonical.com,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        linux-coco@lists.linux.dev, linux-efi <linux-efi@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCHv7 00/14] mm, x86/cc: Implement support for unaccepted
 memory
Message-ID: <20220718172159.4vwjzrfthelovcty@black.fi.intel.com>
References: <20220614120231.48165-1-kirill.shutemov@linux.intel.com>
 <CAMkAt6osbEGBFrgn=y1=x4mDHC1aL40BwaW0NdGHF8qmWd7ktA@mail.gmail.com>
 <20220627113019.3q62luiay7izhehr@black.fi.intel.com>
 <CAMj1kXHD5XBAS1aBjzg1RCeK6qgtanUSED_xyTZ0v1j+UShMKw@mail.gmail.com>
 <20220627122230.7eetepoufd5w3lxd@black.fi.intel.com>
 <CAMkAt6oJJaRM_dy=y2BP99VziPriVuA4jAmMc=G7njwJYKFgyg@mail.gmail.com>
 <CAMj1kXHpS2B9Q7AaQ1euGidZUEyR6gfi=e+t1J_Cr8bmK_9mTw@mail.gmail.com>
 <20220627223808.ihgy3epdx6ofll43@black.fi.intel.com>
 <CAMj1kXEdS9SzFZZ4WGH6sR0WDCOgYDZ3Geg6X2sqSnQ-CXXpZA@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAMj1kXEdS9SzFZZ4WGH6sR0WDCOgYDZ3Geg6X2sqSnQ-CXXpZA@mail.gmail.com>
Precedence: bulk

On Tue, Jun 28, 2022 at 07:17:00PM +0200, Ard Biesheuvel wrote:
> On Tue, 28 Jun 2022 at 00:38, Kirill A. Shutemov
> <kirill.shutemov@linux.intel.com> wrote:
> >
> > On Mon, Jun 27, 2022 at 06:33:51PM +0200, Ard Biesheuvel wrote:
> > > > > > >
> > > > > > > Just as an idea, we can put info into UTS_VERSION which can be read from
> > > > > > > the built bzImage. We have info on SMP and preeption there already.
> > > > > > >
> > > > > >
> > > > > > Instead of hacking this into the binary, couldn't we define a protocol
> > > > > > that the kernel will call from the EFI stub (before EBS()) to identify
> > > > > > itself as an image that understands unaccepted memory, and knows how
> > > > > > to deal with it?
> > > > > >
> > > > > > That way, the firmware can accept all the memory on behalf of the OS
> > > > > > at ExitBootServices() time, unless the OS has indicated there is no
> > > > > > need to do so.
> > > > >
> > > > > I agree it would be better. But I think it would require change to EFI
> > > > > spec, no?
> > > >
> > > > Could this somehow be amended on to the UEFI Specification version 2.9
> > > > change which added all of the unaccepted memory features?
> > > >
> > >
> > > Why would this need a change in the EFI spec? Not every EFI protocol
> > > needs to be in the spec.
> >
> > My EFI knowledge is shallow. Do we do this in other cases?
> >
> 
> The E in EFI means 'extensible' and the whole design of a protocol
> database using GUIDs as identifiers (which will not collide and
> therefore need no a priori coordination when defining them) is
> intended to allow extensions to be defined and implemented in a
> distributed manner.
> 
> Of course, it would be fantastic if we can converge on a protocol that
> all flavors of confidential compute can use, across different OSes, so
> it is generally good if a protocol is defined in *some* shared
> specification. But this doesn't have to be the EFI spec.

I've talked with our firmware expert today and I think we have a problem
with the approach when kernel declaries support of unaccepted memory.

This apporach doesn't work if we include bootloader into the picture: if
EBS() called by bootloader we still cannot know if target kernel supports
unaccepted memory and we return to the square 1.

I think we should make it obvious from a kernel image if it supports
unaccepted memory (with UTS_VERSION or other way).

Any comments?

-- 
 Kirill A. Shutemov