linux-kernel - Re: [PATCH 1/5] mm: Add support for unaccepted memory

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Date:   Tue, 10 Aug 2021 09:48:04 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        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>
Cc:     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>, x86@...nel.org,
        linux-mm@...ck.org, linux-coco@...ts.linux.dev,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 1/5] mm: Add support for unaccepted memory

On 10.08.21 08:26, Kirill A. Shutemov wrote:
> UEFI Specification version 2.9 introduces concept of memory acceptance:
> Some Virtual Machine platforms, such as Intel TDX or AMD SEV-SNP,
> requiring memory to be accepted before it can be used by the guest.
> Accepting happens via a protocol specific for the Virtrual Machine
> platform.
> 
> Accepting memory is costly and it makes VMM allocate memory for the
> accepted guest physical address range. It's better to postpone memory
> acceptation until memory is needed. It lowers boot time and reduces
> memory overhead.
> 
> Support of such memory requires few changes in core-mm code:
> 
>    - memblock has to accept memory on allocation;
> 
>    - page allocator has to accept memory on the first allocation of the
>      page;
> 
> Memblock change is trivial.
> 
> Page allocator is modified to accept pages on the first allocation.
> PageOffline() is used to indicate that the page requires acceptance.
> The flag currently used by hotplug and balloon. Such pages are not
> available to page allocator.
> 
> An architecture has to provide three helpers if it wants to support
> unaccepted memory:
> 
>   - accept_memory() makes a range of physical addresses accepted.
> 
>   - maybe_set_page_offline() marks a page PageOffline() if it requires
>     acceptance. Used during boot to put pages on free lists.
> 
>   - clear_page_offline() clears makes a page accepted and clears
>     PageOffline().
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>   mm/internal.h   | 14 ++++++++++++++
>   mm/memblock.c   |  1 +
>   mm/page_alloc.c | 13 ++++++++++++-
>   3 files changed, 27 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index 31ff935b2547..d2fc8a17fbe0 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -662,4 +662,18 @@ void vunmap_range_noflush(unsigned long start, unsigned long end);
>   int numa_migrate_prep(struct page *page, struct vm_area_struct *vma,
>   		      unsigned long addr, int page_nid, int *flags);
>   
> +#ifndef CONFIG_UNACCEPTED_MEMORY
> +static inline void maybe_set_page_offline(struct page *page, unsigned int order)
> +{
> +}
> +
> +static inline void clear_page_offline(struct page *page, unsigned int order)
> +{
> +}
> +
> +static inline void accept_memory(phys_addr_t start, phys_addr_t end)
> +{
> +}

Can we find better fitting names for the first two? The function names 
are way too generic. For example:

accept_or_set_page_offline()

accept_and_clear_page_offline()

I thought for a second if
	PAGE_TYPE_OPS(Unaccepted, offline)
makes sense as well, not sure.


Also, please update the description of PageOffline in page-flags.h to 
include the additional usage with PageBuddy set at the same time.


I assume you don't have to worry about page_offline_freeze/thaw ... as 
we only set PageOffline initially, but not later at runtime when other 
subsystems (/proc/kcore) might stumble over it.

-- 
Thanks,

David / dhildenb