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@xxxxxxxx> 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(). -- Catalin
