Re: [PATCHv4 1/8] mm: Add support for unaccepted memory

David Hildenbrand
2022-04-08T19:04:22Z

On 06.04.22 01:43, Kirill A. Shutemov wrote:
> UEFI Specification version 2.9 introduces the 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 Virtual 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
> acceptance until memory is needed. It lowers boot time and reduces
> memory overhead.
>
> Support of such memory requires a 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.
>
> The page allocator is modified to accept pages on the first allocation.
> PageUnaccepted() is used to indicate that the page requires acceptance.
>
> Kernel only needs to accept memory once after boot, so during the boot
> and warm up phase there will be a lot of memory acceptance. After things
> are settled down the only price of the feature if couple of checks for
> PageUnaccepted() in allocate and free paths. The check refers a hot
> variable (that also encodes PageBuddy()), so it is cheap and not visible
> on profiles.
>
> Architecture has to provide two helpers if it wants to support
> unaccepted memory:
>
> - accept_memory() makes a range of physical addresses accepted.
>
> - memory_is_unaccepted() checks anything within the range of physical
> addresses requires acceptance.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Acked-by: Mike Rapoport <rppt@linux.ibm.com> # memblock

Thanks, I skimmed over most parts and nothing obvious jumped at me. Dave
has some good suggestions; I'll try giving it a thorough in the near future.

--
Thanks,

David / dhildenb