[PATCHv9 00/14] mm, x86/cc: Implement support for unaccepted memory

Kirill A. Shutemov
2023-03-30T11:51:07Z

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.

The kernel needs to know what memory has been accepted. Firmware
communicates this information via memory map: a new memory type --
EFI_UNACCEPTED_MEMORY -- indicates such memory.

Range-based tracking works fine for firmware, but it gets bulky for
the kernel: e820 has to be modified on every page acceptance. It leads
to table fragmentation, but there's a limited number of entries in the
e820 table

Another option is to mark such memory as usable in e820 and track if the
range has been accepted in a bitmap. One bit in the bitmap represents
2MiB in the address space: one 4k page is enough to track 64GiB or
physical address space.

In the worst-case scenario -- a huge hole in the middle of the
address space -- It needs 256MiB to handle 4PiB of the address
space.

Any unaccepted memory that is not aligned to 2M gets accepted upfront.

The approach lowers boot time substantially. Boot to shell is ~2.5x
faster for 4G TDX VM and ~4x faster for 64G.

TDX-specific code isolated from the core of unaccepted memory support. It
supposed to help to plug-in different implementation of unaccepted memory
such as SEV-SNP.

-- Fragmentation study --

Vlastimil and Mel were concern about effect of unaccepted memory on
fragmentation prevention measures in page allocator. I tried to evaluate
it, but it is tricky. As suggested I tried to run multiple parallel kernel
builds and follow how often kmem:mm_page_alloc_extfrag gets hit.

I don't like the results. Not because unaccepted memory is slow, but
because the test is way too noisy to produce sensible information.

So, I run 8 parallel builds of kernel, 16 jobs each in a VM with 16 CPU
and 16G of RAM. I compared unpatched base-line (mm-unstable) with the tree
that has patchset applied. For the later case, all memory above 1G was
considered unaccepted with fake_unaccepted_start=1G. Around 14G of RAM was
accounted as unaccepted after the boot. The test got all memory accepted.

kmem:mm_page_alloc_extfrag Time elapsed
Base line:
Run 1 837 1803.21s
Run 2 3,845 1785.87s
Run 3 1,704 1883.59s

Patched:
Run 1 905 1876.02s
Run 2 845 1758.50s
Run 3 1276 1876.13s

As you can see the numbers are all over the place.

The good news is that unaccepted memory doesn't make picture notably worse.

I am open to suggestions on how to test it better.

Also, feel free to play with it yourself. fake_unaccepted_start= doesn't
require any special setup.

--

The tree can be found here:

https://github.com/intel/tdx.git guest-unaccepted-memory

The patchset depends on MAX_ORDER changes in MM tree.

v9:
- Accept memory up to high watermark when kernel runs out of free memory;
- Treat unaccepted memory as unusable in __zone_watermark_unusable_free();
- Per-zone unaccepted memory accounting;
- All pages on unaccepted list are MAX_ORDER now;
- accept_memory=eager in cmdline to pre-accept memory during the boot;
- Implement fake unaccepted memory;
- Sysfs handle to accept memory manually;
- Drop PageUnaccepted();
- Rename unaccepted_pages static key to zones_with_unaccepted_pages;
v8:
- Rewrite core-mm support for unaccepted memory (patch 02/14);
- s/UnacceptedPages/Unaccepted/ in meminfo;
- Drop arch/x86/boot/compressed/compiler.h;
- Fix build errors;
- Adjust commit messages and comments;
- Reviewed-bys from Dave and Borislav;
- Rebased to tip/master.
v7:
- Rework meminfo counter to use PageUnaccepted() and move to generic code;
- Fix range_contains_unaccepted_memory() on machines without unaccepted memory;
- Add Reviewed-by from David;
v6:
- Fix load_unaligned_zeropad() on machine with unaccepted memory;
- Clear PageUnaccepted() on merged pages, leaving it only on head;
- Clarify error handling in allocate_e820();
- Fix build with CONFIG_UNACCEPTED_MEMORY=y, but without TDX;
- Disable kexec at boottime instead of build conflict;
- Rebased to tip/master;
- Spelling fixes;
- Add Reviewed-by from Mike and David;
v5:
- Updates comments and commit messages;
+ Explain options for unaccepted memory handling;
- Expose amount of unaccepted memory in /proc/meminfo
- Adjust check in page_expected_state();
- Fix error code handling in allocate_e820();
- Centralize __pa()/__va() definitions in the boot stub;
- Avoid includes from the main kernel in the boot stub;
- Use an existing hole in boot_param for unaccepted_memory, instead of adding
to the end of the structure;
- Extract allocate_unaccepted_memory() form allocate_e820();
- Complain if there's unaccepted memory, but kernel does not support it;
- Fix vmstat counter;
- Split up few preparatory patches;
- Random readability adjustments;
v4:
- PageBuddyUnaccepted() -> PageUnaccepted;
- Use separate page_type, not shared with offline;
- Rework interface between core-mm and arch code;
- Adjust commit messages;
- Ack from Mike;

Kirill A. Shutemov (14):
x86/boot: Centralize __pa()/__va() definitions
mm: Add support for unaccepted memory
mm/page_alloc: Fake unaccepted memory
mm/page_alloc: Add sysfs handle to accept accept_memory
efi/x86: Get full memory map in allocate_e820()
x86/boot: Add infrastructure required for unaccepted memory support
efi/x86: Implement support for unaccepted memory
x86/boot/compressed: Handle unaccepted memory
x86/mm: Reserve unaccepted memory bitmap
x86/mm: Provide helpers for unaccepted memory
x86/mm: Avoid load_unaligned_zeropad() stepping into unaccepted memory
x86/tdx: Make _tdx_hypercall() and __tdx_module_call() available in
boot stub
x86/tdx: Refactor try_accept_one()
x86/tdx: Add unaccepted memory support

Documentation/x86/zero-page.rst | 1 +
arch/x86/Kconfig | 2 +
arch/x86/boot/bitops.h | 40 ++++
arch/x86/boot/compressed/Makefile | 3 +-
arch/x86/boot/compressed/align.h | 14 ++
arch/x86/boot/compressed/bitmap.c | 43 ++++
arch/x86/boot/compressed/bitmap.h | 49 +++++
arch/x86/boot/compressed/bits.h | 36 ++++
arch/x86/boot/compressed/efi.h | 1 +
arch/x86/boot/compressed/error.c | 19 ++
arch/x86/boot/compressed/error.h | 1 +
arch/x86/boot/compressed/find.c | 54 +++++
arch/x86/boot/compressed/find.h | 79 ++++++++
arch/x86/boot/compressed/ident_map_64.c | 8 -
arch/x86/boot/compressed/kaslr.c | 35 ++--
arch/x86/boot/compressed/math.h | 37 ++++
arch/x86/boot/compressed/mem.c | 122 +++++++++++
arch/x86/boot/compressed/minmax.h | 61 ++++++
arch/x86/boot/compressed/misc.c | 6 +
arch/x86/boot/compressed/misc.h | 15 ++
arch/x86/boot/compressed/pgtable_types.h | 25 +++
arch/x86/boot/compressed/sev.c | 2 -
arch/x86/boot/compressed/tdx-shared.c | 2 +
arch/x86/boot/compressed/tdx.c | 39 ++++
arch/x86/coco/tdx/Makefile | 2 +-
arch/x86/coco/tdx/tdx-shared.c | 95 +++++++++
arch/x86/coco/tdx/tdx.c | 118 +----------
arch/x86/include/asm/page.h | 3 +
arch/x86/include/asm/shared/tdx.h | 53 +++++
arch/x86/include/asm/tdx.h | 21 +-
arch/x86/include/asm/unaccepted_memory.h | 16 ++
arch/x86/include/uapi/asm/bootparam.h | 2 +-
arch/x86/kernel/e820.c | 17 ++
arch/x86/mm/Makefile | 2 +
arch/x86/mm/unaccepted_memory.c | 107 ++++++++++
drivers/base/node.c | 7 +
drivers/firmware/efi/Kconfig | 14 ++
drivers/firmware/efi/efi.c | 1 +
drivers/firmware/efi/libstub/x86-stub.c | 98 +++++++--
fs/proc/meminfo.c | 5 +
include/linux/efi.h | 3 +-
include/linux/mmzone.h | 8 +
mm/internal.h | 13 ++
mm/memblock.c | 9 +
mm/mm_init.c | 7 +
mm/page_alloc.c | 246 +++++++++++++++++++++++
mm/vmstat.c | 3 +
47 files changed, 1368 insertions(+), 176 deletions(-)
create mode 100644 arch/x86/boot/compressed/align.h
create mode 100644 arch/x86/boot/compressed/bitmap.c
create mode 100644 arch/x86/boot/compressed/bitmap.h
create mode 100644 arch/x86/boot/compressed/bits.h
create mode 100644 arch/x86/boot/compressed/find.c
create mode 100644 arch/x86/boot/compressed/find.h
create mode 100644 arch/x86/boot/compressed/math.h
create mode 100644 arch/x86/boot/compressed/mem.c
create mode 100644 arch/x86/boot/compressed/minmax.h
create mode 100644 arch/x86/boot/compressed/pgtable_types.h
create mode 100644 arch/x86/boot/compressed/tdx-shared.c
create mode 100644 arch/x86/coco/tdx/tdx-shared.c
create mode 100644 arch/x86/include/asm/unaccepted_memory.h
create mode 100644 arch/x86/mm/unaccepted_memory.c

--
2.39.2