Messages in this thread Patch in this message | | | From | Yangyu Chen <> | Subject | [PATCH 3/3] Documentation: riscv: correct sv57 kernel behavior | Date | Mon, 15 Jan 2024 03:58:32 +0800 |
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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@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
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