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diff --git a/Documentation/x86/x86_64/5level-paging.rst b/Documentation/x86/x86_64/5level-paging.rst deleted file mode 100644 index b792bbdc0b01..000000000000 --- a/Documentation/x86/x86_64/5level-paging.rst +++ /dev/null @@ -1,67 +0,0 @@ -.. SPDX-License-Identifier: GPL-2.0 - -============== -5-level paging -============== - -Overview -======== -Original x86-64 was limited by 4-level paging to 256 TiB of virtual address -space and 64 TiB of physical address space. We are already bumping into -this limit: some vendors offer servers with 64 TiB of memory today. - -To overcome the limitation upcoming hardware will introduce support for -5-level paging. It is a straight-forward extension of the current page -table structure adding one more layer of translation. - -It bumps the limits to 128 PiB of virtual address space and 4 PiB of -physical address space. This "ought to be enough for anybody" ©. - -QEMU 2.9 and later support 5-level paging. - -Virtual memory layout for 5-level paging is described in -Documentation/x86/x86_64/mm.rst - - -Enabling 5-level paging -======================= -CONFIG_X86_5LEVEL=y enables the feature. - -Kernel with CONFIG_X86_5LEVEL=y still able to boot on 4-level hardware. -In this case additional page table level -- p4d -- will be folded at -runtime. - -User-space and large virtual address space -========================================== -On x86, 5-level paging enables 56-bit userspace virtual address space. -Not all user space is ready to handle wide addresses. It's known that -at least some JIT compilers use higher bits in pointers to encode their -information. It collides with valid pointers with 5-level paging and -leads to crashes. - -To mitigate this, we are not going to allocate virtual address space -above 47-bit by default. - -But userspace can ask for allocation from full address space by -specifying hint address (with or without MAP_FIXED) above 47-bits. - -If hint address set above 47-bit, but MAP_FIXED is not specified, we try -to look for unmapped area by specified address. If it's already -occupied, we look for unmapped area in *full* address space, rather than -from 47-bit window. - -A high hint address would only affect the allocation in question, but not -any future mmap()s. - -Specifying high hint address on older kernel or on machine without 5-level -paging support is safe. The hint will be ignored and kernel will fall back -to allocation from 47-bit address space. - -This approach helps to easily make application's memory allocator aware -about large address space without manually tracking allocated virtual -address space. - -One important case we need to handle here is interaction with MPX. -MPX (without MAWA extension) cannot handle addresses above 47-bit, so we -need to make sure that MPX cannot be enabled we already have VMA above -the boundary and forbid creating such VMAs once MPX is enabled. |