Re: [PATCHv11 1/9] mm: Add support for unaccepted memory

[Tom Lendacky]

On 5/13/23 17:04, 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, require memory to be accepted before it can be used by the
> guest. Accepting happens via a protocol specific to the Virtual Machine
> platform.
>
> There are several ways kernel can deal with unaccepted memory:
>
>   1. Accept all the memory during the boot. It is easy to implement and
>      it doesn't have runtime cost once the system is booted. The downside
>      is very long boot time.
>
>      Accept can be parallelized to multiple CPUs to keep it manageable
>      (i.e. via DEFERRED_STRUCT_PAGE_INIT), but it tends to saturate
>      memory bandwidth and does not scale beyond the point.
>
>   2. Accept a block of memory on the first use. It requires more
>      infrastructure and changes in page allocator to make it work, but
>      it provides good boot time.
>
>      On-demand memory accept means latency spikes every time kernel steps
>      onto a new memory block. The spikes will go away once workload data
>      set size gets stabilized or all memory gets accepted.
>
>   3. Accept all memory in background. Introduce a thread (or multiple)
>      that gets memory accepted proactively. It will minimize time the
>      system experience latency spikes on memory allocation while keeping
>      low boot time.
>
>      This approach cannot function on its own. It is an extension of #2:
>      background memory acceptance requires functional scheduler, but the
>      page allocator may need to tap into unaccepted memory before that.
>
>      The downside of the approach is that these threads also steal CPU
>      cycles and memory bandwidth from the user's workload and may hurt
>      user experience.
>
> The patch implements #1 and #2 for now. #2 is the default. Some
> workloads may want to use #1 with accept_memory=eager in kernel
> command line. #3 can be implemented later based on user's demands.
>
> Support of unaccepted 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. New memory gets accepted
> before putting pages on free lists. It is done lazily: only accept new
> pages when we run out of already accepted memory. The memory gets
> accepted until the high watermark is reached.
>
> EFI code will provide two helpers if the platform supports unaccepted
> memory:
>
>   - accept_memory() makes a range of physical addresses accepted.
>
>   - range_contains_unaccepted_memory() checks anything within the range
>     of physical addresses requires acceptance.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Acked-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>	# memblock
> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
>   drivers/base/node.c    |   7 ++
>   fs/proc/meminfo.c      |   5 ++
>   include/linux/mm.h     |  19 +++++
>   include/linux/mmzone.h |   8 ++
>   mm/internal.h          |   1 +
>   mm/memblock.c          |   9 +++
>   mm/mm_init.c           |   7 ++
>   mm/page_alloc.c        | 173 +++++++++++++++++++++++++++++++++++++++++
>   mm/vmstat.c            |   3 +
>   9 files changed, 232 insertions(+)

> diff --git a/mm/internal.h b/mm/internal.h
> index 68410c6d97ac..b1db7ba5f57d 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -1099,4 +1099,5 @@ struct vma_prepare {
>   	struct vm_area_struct *remove;
>   	struct vm_area_struct *remove2;
>   };
> +

Looks like an unintentional change.

>   #endif	/* __MM_INTERNAL_H */
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 3feafea06ab2..50b921119600 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -1436,6 +1436,15 @@ phys_addr_t __init memblock_alloc_range_nid(phys_addr_t size,
>   		 */
>   		kmemleak_alloc_phys(found, size, 0);
>   
> +	/*
> +	 * Some Virtual Machine platforms, such as Intel TDX or AMD SEV-SNP,
> +	 * require memory to be accepted before it can be used by the
> +	 * guest.
> +	 *
> +	 * Accept the memory of the allocated buffer.
> +	 */
> +	accept_memory(found, found + size);

I'm not an mm or memblock expert, but do we need to worry about freed 
memory from memblock_phys_free() being possibly doubly accepted? A double 
acceptance will trigger a guest termination on SNP.

Thanks,
Tom

> +
>   	return found;
>   }
>