From: Dave Hansen <dave.hansen@intel.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.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>, Ard Biesheuvel <ardb@kernel.org>,
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>,
x86@kernel.org, linux-mm@kvack.org, linux-coco@lists.linux.dev,
linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCHv2 5/7] x86/mm: Reserve unaccepted memory bitmap
Date: Wed, 12 Jan 2022 11:53:42 -0800 [thread overview]
Message-ID: <a4b5707f-0f0b-57a8-ccdb-d89f66210b52@intel.com> (raw)
In-Reply-To: <20220112194302.cyxhjypsptr4mtix@box.shutemov.name>
On 1/12/22 11:43 AM, Kirill A. Shutemov wrote:
> On Tue, Jan 11, 2022 at 11:10:40AM -0800, Dave Hansen wrote:
>> On 1/11/22 03:33, Kirill A. Shutemov wrote:
>>> Unaccepted memory bitmap is allocated during decompression stage and
>>> handed over to main kernel image via boot_params. The bitmap is used to
>>> track if memory has been accepted.
>>>
>>> Reserve unaccepted memory bitmap has to prevent reallocating memory for
>>> other means.
>>
>> I'm having a hard time parsing that changelog, especially the second
>> paragraph. Could you give it another shot?
>
> What about this:
>
> Unaccepted memory bitmap is allocated during decompression stage and
> handed over to main kernel image via boot_params.
>
> Kernel tracks what memory has been accepted in the bitmap.
>
> Reserve memory where the bitmap is placed to prevent memblock from
> re-allocating the memory for other needs.
>
> ?
Ahh, I get what you're trying to say now. But, it still really lacks a
coherent problem statement. How about this?
== Problem ==
A given page of memory can only be accepted once. The kernel
has a need to accept memory both in the early decompression
stage and during normal runtime.
== Solution ==
Use a bitmap to communicate the acceptance state of each page
between the decompression stage and normal runtime. This
eliminates the possibility of attempting to double-accept a
page.
== Details ==
Allocate the bitmap during decompression stage and hand it over
to the main kernel image via boot_params.
In the runtime kernel, reserve the bitmap's memory to ensure
nothing overwrites it.
>>> + /* Mark unaccepted memory bitmap reserved */
>>> + if (boot_params.unaccepted_memory) {
>>> + unsigned long size;
>>> +
>>> + /* One bit per 2MB */
>>> + size = DIV_ROUND_UP(e820__end_of_ram_pfn() * PAGE_SIZE,
>>> + PMD_SIZE * BITS_PER_BYTE);
>>> + memblock_reserve(boot_params.unaccepted_memory, size);
>>> + }
>>
>> Is it OK that the size of the bitmap is inferred from
>> e820__end_of_ram_pfn()? Is this OK in the presence of mem= and other things
>> that muck with the e820?
>
> Good question. I think we are fine. If kernel is not able to allocate
> memory from a part of physical address space we don't need the bitmap for
> it either.
That's a good point. If the e820 range does a one-way shrink it's
probably fine. The only problem would be if the bitmap had space for
for stuff past e820__end_of_ram_pfn() *and* it later needed to be accepted.
Would it be worth recording the size of the reservation and then
double-checking against it in the bitmap operations?
next prev parent reply other threads:[~2022-01-12 19:56 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-11 11:33 [PATCHv2 0/7] Implement support for unaccepted memory Kirill A. Shutemov
2022-01-11 11:33 ` [PATCHv2 1/7] mm: Add " Kirill A. Shutemov
2022-01-11 19:46 ` Dave Hansen
2022-01-12 11:31 ` David Hildenbrand
2022-01-12 19:15 ` Kirill A. Shutemov
2022-01-14 13:22 ` David Hildenbrand
2022-01-12 18:30 ` Kirill A. Shutemov
2022-01-12 18:40 ` Dave Hansen
2022-01-13 7:42 ` Mike Rapoport
2022-01-11 11:33 ` [PATCHv2 2/7] efi/x86: Get full memory map in allocate_e820() Kirill A. Shutemov
2022-01-11 11:33 ` [PATCHv2 3/7] efi/x86: Implement support for unaccepted memory Kirill A. Shutemov
2022-01-11 17:17 ` Dave Hansen
2022-01-12 19:29 ` Kirill A. Shutemov
2022-01-12 19:35 ` Dave Hansen
2022-01-11 11:33 ` [PATCHv2 4/7] x86/boot/compressed: Handle " Kirill A. Shutemov
2022-01-11 11:33 ` [PATCHv2 5/7] x86/mm: Reserve unaccepted memory bitmap Kirill A. Shutemov
2022-01-11 19:10 ` Dave Hansen
2022-01-12 19:43 ` Kirill A. Shutemov
2022-01-12 19:53 ` Dave Hansen [this message]
2022-01-15 18:46 ` Mike Rapoport
2022-01-11 11:33 ` [PATCHv2 6/7] x86/mm: Provide helpers for unaccepted memory Kirill A. Shutemov
2022-01-11 20:01 ` Dave Hansen
2022-01-12 19:43 ` Kirill A. Shutemov
2022-01-11 11:33 ` [PATCHv2 7/7] x86/tdx: Unaccepted memory support Kirill A. Shutemov
2022-01-18 21:05 ` [PATCHv2 0/7] Implement support for unaccepted memory Brijesh Singh
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