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Message-ID: <20230708011156.2697409-5-charlie@rivosinc.com>
Date: Fri, 7 Jul 2023 18:11:35 -0700
From: Charlie Jenkins <charlie@...osinc.com>
To: linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc: charlie@...osinc.com, conor@...nel.org, paul.walmsley@...ive.com,
palmer@...osinc.com, aou@...s.berkeley.edu, anup@...infault.org,
konstantin@...uxfoundation.org, linux-doc@...r.kernel.org,
linux-kselftest@...r.kernel.org, linux-mm@...ck.org,
mick@....forth.gr, jrtc27@...c27.com, rdunlap@...radead.org
Subject: [PATCH v4 4/4] RISC-V: mm: Document mmap changes
The behavior of mmap is modified with this patch series, so explain the
changes to the mmap hint address behavior.
Signed-off-by: Charlie Jenkins <charlie@...osinc.com>
---
Documentation/riscv/vm-layout.rst | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)
diff --git a/Documentation/riscv/vm-layout.rst b/Documentation/riscv/vm-layout.rst
index 5462c84f4723..8141addbf888 100644
--- a/Documentation/riscv/vm-layout.rst
+++ b/Documentation/riscv/vm-layout.rst
@@ -133,3 +133,25 @@ RISC-V Linux Kernel SV57
ffffffff00000000 | -4 GB | ffffffff7fffffff | 2 GB | modules, BPF
ffffffff80000000 | -2 GB | ffffffffffffffff | 2 GB | kernel
__________________|____________|__________________|_________|____________________________________________________________
+
+
+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 other VA space by providing
+a hint address to mmap. A call to mmap is guaranteed to return an address
+that will not override the unset left-aligned bits in the hint address,
+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 << 38` must be provided. Note that this is 38 due to sv39 userspace
+ending at :code:`1 << 38` 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 << 47` must be provided.
--
2.41.0
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