在2024年9月5日九月 下午10:15，Charlie Jenkins写道：

Some applications rely on placing data in free bits addresses allocated
by mmap. Various architectures (eg. x86, arm64, powerpc) restrict the
address returned by mmap to be less than the 48-bit address space,
unless the hint address uses more than 47 bits (the 48th bit is reserved
for the kernel address space).

The riscv architecture needs a way to similarly restrict the virtual
address space. On the riscv port of OpenJDK an error is thrown if
attempted to run on the 57-bit address space, called sv57 [1].  golang
has a comment that sv57 support is not complete, but there are some
workarounds to get it to mostly work [2].

These applications work on x86 because x86 does an implicit 47-bit
restriction of mmap() address that contain a hint address that is less
than 48 bits.

Instead of implicitly restricting the address space on riscv (or any
current/future architecture), provide a flag to the personality syscall
that can be used to ensure an application works in any arbitrary VA
space. A similar feature has already been implemented by the personality
syscall in ADDR_LIMIT_32BIT.

This flag will also allow seemless compatibility between all
architectures, so applications like Go and OpenJDK that use bits in a
virtual address can request the exact number of bits they need in a
generic way. The flag can be checked inside of vm_unmapped_area() so
that this flag does not have to be handled individually by each
architecture. 

Tested-by: Jiaxun Yang <jiaxun.yang@flygoat.com>

Tested on MIPS VA 48 system, fixed pointer tagging on mozjs!

Thanks!

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- Jiaxun