On Tue, Sep 10, 2024 at 05:45:07PM -0700, Charlie Jenkins wrote: > On Tue, Sep 10, 2024 at 03:08:14PM -0400, Liam R. Howlett wrote: > > * Catalin Marinas <catalin.marinas@xxxxxxx> [240906 07:44]: > > > 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() Point 4 is an ABI change. What guarantees that there isn't still software out there that relies on the old behaviour? > > > > 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(). > > > > I think that is best as well. > > > > Can we please just do what x86 and arm64 does? > > I responded to Arnd in the other thread, but I am still not convinced > that the solution that x86 and arm64 have selected is the best solution. > The solution of defaulting to 47 bits does allow applications the > ability to get addresses that are below 47 bits. However, due to > differences across architectures it doesn't seem possible to have all > architectures default to the same value. Additionally, this flag will be > able to help users avoid potential bugs where a hint address is passed > that causes upper bits of a VA to be used. The reason we added this limit on arm64 is that we noticed programs using the top 8 bits of a 64-bit pointer for additional information. IIRC, it wasn't even openJDK but some JavaScript JIT. We could have taught those programs of a new flag but since we couldn't tell how many are out there, it was the safest to default to a smaller limit and opt in to the higher one. Such opt-in is via mmap() but if you prefer a prctl() flag, that's fine by me as well (though I think this should be opt-in to higher addresses rather than opt-out of the higher addresses). -- Catalin
