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

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Date:   Wed, 1 Jun 2022 16:39:16 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
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 01.06.22 16:35, Kirill A. Shutemov wrote:
> 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.

Essentially, if the PFN you calculate here for the bitmap size will
essentially match later max_pfn.

> 
> 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.
> 


-- 
Thanks,

David / dhildenb