On Fri, Sep 06, 2024 at 09:55:42AM +0000, Arnd Bergmann wrote:
On Fri, Sep 6, 2024, at 09:14, Guo Ren wrote:
On Fri, Sep 6, 2024 at 3:18 PM Arnd Bergmann arnd@arndb.de wrote:
It's also unclear to me how we want this flag to interact with the existing logic in arch_get_mmap_end(), which attempts to limit the default mapping to a 47-bit address space already.
To optimize RISC-V progress, I recommend:
Step 1: Approve the patch. Step 2: Update Go and OpenJDK's RISC-V backend to utilize it. Step 3: Wait approximately several iterations for Go & OpenJDK Step 4: Remove the 47-bit constraint in arch_get_mmap_end()
I really want to first see a plausible explanation about why RISC-V can't just implement this using a 47-bit DEFAULT_MAP_WINDOW like all the other major architectures (x86, arm64, powerpc64),
FWIW arm64 actually limits DEFAULT_MAP_WINDOW to 48-bit in the default configuration. We end up with a 47-bit with 16K pages but for a different reason that has to do with LPA2 support (I doubt we need this for the user mapping but we need to untangle some of the macros there; that's for a separate discussion).
That said, we haven't encountered any user space problems with a 48-bit DEFAULT_MAP_WINDOW. So I also think RISC-V should follow a similar approach (47 or 48 bit default limit). Better to have some ABI consistency between architectures. One can still ask for addresses above this default limit via mmap().