linux-kernel - Re: [PATCHv6 03/15] efi/x86: Get full memory map in allocate

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Message-ID: <20220601143515.iavmtysdchirbtel@box.shutemov.name>
Date:   Wed, 1 Jun 2022 17:35:15 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     David Hildenbrand <david@...hat.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>,
        Ard Biesheuvel <ardb@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Varad Gautam <varad.gautam@...e.com>,
        Dario Faggioli <dfaggioli@...e.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Mike Rapoport <rppt@...nel.org>, marcelo.cerri@...onical.com,
        tim.gardner@...onical.com, khalid.elmously@...onical.com,
        philip.cox@...onical.com, x86@...nel.org, linux-mm@...ck.org,
        linux-coco@...ts.linux.dev, linux-efi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv6 03/15] efi/x86: Get full memory map in allocate_e820()

On Wed, Jun 01, 2022 at 11:00:23AM +0200, David Hildenbrand wrote:
> On 17.05.22 17:34, Kirill A. Shutemov wrote:
> > Currently allocate_e820() only interested in the size of map and size of
> > memory descriptor to determine how many e820 entries the kernel needs.
> > 
> > UEFI Specification version 2.9 introduces a new memory type --
> > unaccepted memory. To track unaccepted memory kernel needs to allocate
> > a bitmap. The size of the bitmap is dependent on the maximum physical
> > address present in the system. A full memory map is required to find
> > the maximum address.
> > 
> > Modify allocate_e820() to get a full memory map.
> 
> Usually we use max_pfn, if we want to know the maximum pfn that's
> present in the system (well, IIRC, excluding hotunplug).
> 
> How exactly will this (different?) maximum from UEFI for the bitmap
> interact with
> 
> max_pfn = e820__end_of_ram_pfn();
> 
> from e820 in existing code
> 
> ?

I'm not sure I understand the question.

On EFI system, E820 is constructed based on EFI memory map and size of
bitmap calculated based of EFI memmap will always be enough to address all
memory. e820__end_of_ram_pfn() can be smaller than what what we calculate
as size of memory here, if kernel reserve very top of the memory, but it
will never be larger.

Later during the boot we use e820__end_of_ram_pfn() to infer size of
bitmap and it is safe.

-- 
 Kirill A. Shutemov