x86: Enable 5-level paging support via CONFIG_X86_5LEVEL=y

Most of things are in place and we can enable support for 5-level paging.

The patch makes XEN_PV and XEN_PVH dependent on !X86_5LEVEL. Both are
not ready to work with 5-level paging.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arch@vger.kernel.org
Cc: linux-mm@kvack.org
Link: http://lkml.kernel.org/r/20170716225954.74185-9-kirill.shutemov@linux.intel.com
[ Minor readability edits. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 64 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 0000000..087251a
--- /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.
+