From: Charlie Jenkins <charlie@rivosinc.com>
To: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
Cc: charlie@rivosinc.com, conor@kernel.org, paul.walmsley@sifive.com,
palmer@rivosinc.com, aou@eecs.berkeley.edu, anup@brainfault.org,
konstantin@linuxfoundation.org, linux-doc@vger.kernel.org,
linux-kselftest@vger.kernel.org, linux-mm@kvack.org,
mick@ics.forth.gr, jrtc27@jrtc27.com, rdunlap@infradead.org,
alexghiti@rivosinc.com
Subject: [PATCH v10 4/4] RISC-V: mm: Document mmap changes
Date: Wed, 9 Aug 2023 16:22:04 -0700 [thread overview]
Message-ID: <20230809232218.849726-5-charlie@rivosinc.com> (raw)
In-Reply-To: <20230809232218.849726-1-charlie@rivosinc.com>
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@rivosinc.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.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..69ff6da1dbf8 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 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.
--
2.34.1
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next prev parent reply other threads:[~2023-08-09 23:24 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-08-09 23:22 [PATCH v10 0/4] RISC-V: mm: Make SV48 the default address space Charlie Jenkins
2023-08-09 23:22 ` [PATCH v10 1/4] RISC-V: mm: Restrict address space for sv39,sv48,sv57 Charlie Jenkins
2023-08-09 23:22 ` [PATCH v10 2/4] RISC-V: mm: Add tests for RISC-V mm Charlie Jenkins
2023-08-09 23:22 ` [PATCH v10 3/4] RISC-V: mm: Update pgtable comment documentation Charlie Jenkins
2023-08-09 23:22 ` Charlie Jenkins [this message]
2023-08-30 13:20 ` [PATCH v10 0/4] RISC-V: mm: Make SV48 the default address space patchwork-bot+linux-riscv
2024-01-13 17:26 ` Yangyu Chen
2024-01-20 1:34 ` Charlie Jenkins
2024-01-20 6:13 ` Yangyu Chen
2024-01-20 6:49 ` Charlie Jenkins
2024-01-20 7:09 ` Yangyu Chen
2024-01-22 19:56 ` Charlie Jenkins
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