Documentation/arm64: HugeTLB page implementation

Arm v8 architecture supports multiple page sizes - 4k, 16k and
64k. Based on the active page size, the Linux port supports
corresponding hugepage sizes at PMD and PUD(4k only) levels.

In addition, the architecture also supports caching larger sized
ranges (composed of multiple entries) at the PTE and PMD level in the
TLBs using the contiguous bit. The Linux port makes use of this
architectural support to enable additional hugepage sizes.

Describe the two different types of hugepages supported by the arm64
kernel and the hugepage sizes enabled by each.

Acked-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/Documentation/arm64/hugetlbpage.txt b/Documentation/arm64/hugetlbpage.txt
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+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