diff options
| -rw-r--r-- | Documentation/x86/x86_64/5level-paging.txt | 64 | ||||
| -rw-r--r-- | arch/x86/Kconfig | 19 | ||||
| -rw-r--r-- | arch/x86/xen/Kconfig | 5 |
3 files changed, 88 insertions, 0 deletions
diff --git a/Documentation/x86/x86_64/5level-paging.txt b/Documentation/x86/x86_64/5level-paging.txt new file mode 100644 index 000000000000..087251a0d99c --- /dev/null +++ b/Documentation/x86/x86_64/5level-paging.txt @@ -0,0 +1,64 @@ +== Overview == + +Original x86-64 was limited by 4-level paing to 256 TiB of virtual address +space and 64 TiB of physical address space. We are already bumping into +this limit: some vendors offers 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.txt + +== Enabling 5-level paging == + +CONFIG_X86_5LEVEL=y enables the feature. + +So far, a kernel compiled with the option enabled will be able to boot +only on machines that supports the feature -- see for 'la57' flag in +/proc/cpuinfo. + +The plan is to implement boot-time switching between 4- and 5-level paging +in the future. + +== 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. + diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index 8328bcb9ce8b..ff637dedfafa 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -326,6 +326,7 @@ config FIX_EARLYCON_MEM config PGTABLE_LEVELS int + default 5 if X86_5LEVEL default 4 if X86_64 default 3 if X86_PAE default 2 @@ -1398,6 +1399,24 @@ config X86_PAE has the cost of more pagetable lookup overhead, and also consumes more pagetable space per process. +config X86_5LEVEL + bool "Enable 5-level page tables support" + depends on X86_64 + ---help--- + 5-level paging enables access to larger address space: + upto 128 PiB of virtual address space and 4 PiB of + physical address space. + + It will be supported by future Intel CPUs. + + Note: a kernel with this option enabled can only be booted + on machines that support the feature. + + See Documentation/x86/x86_64/5level-paging.txt for more + information. + + Say N if unsure. + config ARCH_PHYS_ADDR_T_64BIT def_bool y depends on X86_64 || X86_PAE diff --git a/arch/x86/xen/Kconfig b/arch/x86/xen/Kconfig index 027987638e98..1ecd419811a2 100644 --- a/arch/x86/xen/Kconfig +++ b/arch/x86/xen/Kconfig @@ -17,6 +17,9 @@ config XEN_PV bool "Xen PV guest support" default y depends on XEN + # XEN_PV is not ready to work with 5-level paging. + # Changes to hypervisor are also required. + depends on !X86_5LEVEL select XEN_HAVE_PVMMU select XEN_HAVE_VPMU help @@ -75,4 +78,6 @@ config XEN_DEBUG_FS config XEN_PVH bool "Support for running as a PVH guest" depends on XEN && XEN_PVHVM && ACPI + # Pre-built page tables are not ready to handle 5-level paging. + depends on !X86_5LEVEL def_bool n |