[PATCH 3/3] Documentation: riscv: correct sv57 kernel behavior

[Yangyu Chen]

The original documentation from a patch violates the principle of mmap.
Since the kernel behavior has been corrected from the previous patch, this
documentation should also be updated. This patch migrated the
5-level-paging documentation from x86_64 with minor modifications to align
with the current kernel's behavior on RISC-V.

Signed-off-by: Yangyu Chen <cyy at cyyself.name>
---
 Documentation/arch/riscv/vm-layout.rst | 48 +++++++++++++++-----------
 1 file changed, 28 insertions(+), 20 deletions(-)

diff --git a/Documentation/arch/riscv/vm-layout.rst b/Documentation/arch/riscv/vm-layout.rst
index 69ff6da1dbf8..30e879dad6a2 100644
--- a/Documentation/arch/riscv/vm-layout.rst
+++ b/Documentation/arch/riscv/vm-layout.rst
@@ -135,23 +135,31 @@ RISC-V Linux Kernel SV57
   __________________|____________|__________________|_________|____________________________________________________________
 
 
-Userspace VAs
---------------------
-To maintain compatibility with software that relies on the VA space with a
-maximum of 48 bits the kernel will, by default, return virtual addresses to
-userspace from a 48-bit range (sv48). This default behavior is achieved by
-passing 0 into the hint address parameter of mmap. On CPUs with an address space
-smaller than sv48, the CPU maximum supported address space will be the default.
-
-Software can "opt-in" to receiving VAs from another VA space by providing
-a hint address to mmap. A hint address passed to mmap will cause the largest
-address space that fits entirely into the hint to be used, unless there is no
-space left in the address space. If there is no space available in the requested
-address space, an address in the next smallest available address space will be
-returned.
-
-For example, in order to obtain 48-bit VA space, a hint address greater than
-:code:`1 << 47` must be provided. Note that this is 47 due to sv48 userspace
-ending at :code:`1 << 47` and the addresses beyond this are reserved for the
-kernel. Similarly, to obtain 57-bit VA space addresses, a hint address greater
-than or equal to :code:`1 << 56` must be provided.
+User-space and large virtual address space
+==========================================
+On RISC-V, Sv57 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 Sv57 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,
+this mmap will fail.
+
+A high hint address would only affect the allocation in question, but not
+any future mmap()s.
+
+Specifying high hint address without MAP_FIXED on older kernel or on
+machine without Sv57 paging support is safe. The hint will be ignored and
+kernel will fall back to allocation from the supported address space.
+
+This approach helps to easily make application's memory allocator aware
+about large address space without manually tracking allocated virtual
+address space.
-- 
2.43.0