[mirror_ubuntu-jammy-kernel.git] / Documentation / arm64 / hugetlbpage.rst

.. _hugetlbpage_index:

====================
HugeTLBpage on ARM64
====================

Hugepage relies on making efficient use of TLBs to improve performance of
address translations. The benefit depends on both -

  - the size of hugepages
  - size of entries supported by the TLBs

The ARM64 port supports two flavours of hugepages.

1) Block mappings at the pud/pmd level
--------------------------------------

These are regular hugepages where a pmd or a pud page table entry points to a
block of memory. Regardless of the supported size of entries in TLB, block
mappings reduce the depth of page table walk needed to translate hugepage
addresses.

2) Using the Contiguous bit
---------------------------

The architecture provides a contiguous bit in the translation table entries
(D4.5.3, ARM DDI 0487C.a) that hints to the MMU to indicate that it is one of a
contiguous set of entries that can be cached in a single TLB entry.

The contiguous bit is used in Linux to increase the mapping size at the pmd and
pte (last) level. The number of supported contiguous entries varies by page size
and level of the page table.


The following hugepage sizes are supported -

  ====== ========   ====    ========    ===
  -      CONT PTE    PMD    CONT PMD    PUD
  ====== ========   ====    ========    ===
  4K:         64K     2M         32M     1G
  16K:         2M    32M          1G
  64K:         2M   512M         16G
  ====== ========   ====    ========    ===
Commit	Line	Data
e0533dee BL	1	.. _hugetlbpage_index:
e0533dee BL	2
b693d0b3	3	====================
0c09d485 PA	4	HugeTLBpage on ARM64
	5	====================
	6
	7	Hugepage relies on making efficient use of TLBs to improve performance of
	8	address translations. The benefit depends on both -
	9
	10	- the size of hugepages
	11	- size of entries supported by the TLBs
	12
	13	The ARM64 port supports two flavours of hugepages.
	14
	15	1) Block mappings at the pud/pmd level
	16	--------------------------------------
	17
	18	These are regular hugepages where a pmd or a pud page table entry points to a
	19	block of memory. Regardless of the supported size of entries in TLB, block
	20	mappings reduce the depth of page table walk needed to translate hugepage
	21	addresses.
	22
	23	2) Using the Contiguous bit
	24	---------------------------
	25
	26	The architecture provides a contiguous bit in the translation table entries
	27	(D4.5.3, ARM DDI 0487C.a) that hints to the MMU to indicate that it is one of a
	28	contiguous set of entries that can be cached in a single TLB entry.
	29
	30	The contiguous bit is used in Linux to increase the mapping size at the pmd and
	31	pte (last) level. The number of supported contiguous entries varies by page size
	32	and level of the page table.
	33
	34
	35	The following hugepage sizes are supported -
	36
b693d0b3 MCC	37	====== ======== ==== ======== ===
	38	- CONT PTE PMD CONT PMD PUD
	39	====== ======== ==== ======== ===
0c09d485 PA	40	4K: 64K 2M 32M 1G
	41	16K: 2M 32M 1G
	42	64K: 2M 512M 16G
b693d0b3	43	====== ======== ==== ======== ===