On Wed, Aug 17, 2016 at 02:54:59PM +0200, Dr. Philipp Tomsich wrote: > On 17 Aug 2016, at 14:48, Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx> wrote: > > On Wed, Aug 17, 2016 at 02:28:50PM +0200, Alexander Graf wrote: > >> On 17 Aug 2016, at 13:46, Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx> wrote: > >>> This series enables aarch64 with ilp32 mode, and as supporting work, > >>> introduces ARCH_32BIT_OFF_T configuration option that is enabled for > >>> existing 32-bit architectures but disabled for new arches (so 64-bit > >>> off_t is is used by new userspace). > >>> > >>> This version is based on kernel v4.8-rc2. > >>> It works with glibc-2.23, and tested with LTP. > >>> > >>> This is RFC because there is still no solid understanding what type of registers > >>> top-halves delousing we prefer. In this patchset, w0-w7 are cleared for each > >>> syscall in assembler entry. The alternative approach is in introducing compat > >>> wrappers which is little faster for natively routed syscalls (~2.6% for syscall > >>> with no payload) but much more complicated. > >> > >> So you’re saying there are 2 options: > >> > >> 1) easy to get right, slightly slower, same ABI to user space as 2 > >> 2) harder to get right, minor performance benefit > > > > No, ABI is little different. If 1) we pass off_t in a pair to syscalls, > > if 2) - in a single register. So if 1, we 'd take some wrappers from aarch32. > > See patch 12 here. > > From our experience with ILP32, I’d prefer to have off_t (and similar) > in a single register whenever possible (i.e. option #2). It feels > more natural to use the full 64bit registers whenever possible, as > ILP32 on ARMv8 should really be understood as a 64bit ABI with a 32bit > memory model. I think we are well past the point where we considered ILP32 a 64-bit ABI. It would have been nice but we decided that breaking POSIX compatibility is a bad idea, so we went back (again) to a 32-bit ABI for ILP32. While there are 64-bit arguments that, at a first look, would make sense to be passed in 64-bit registers, the kernel maintenance cost is significant with changes to generic files. Allowing 64-bit wide registers at the ILP32 syscall interface means that the kernel would have to zero/sign-extend the upper half of the 32-bit arguments for the cases where they are passed directly to a native syscall that expects a 64-bit argument. This (a) adds a significant number of wrappers to the generic code together additional annotations to the generic unistd.h and (b) it adds a small overhead to the AArch32 (compat) ABI since it doesn't need such generic wrapping (the upper half of 64-bit registers is guaranteed to be zero/preserved by the architecture when coming from the AArch32 mode). -- Catalin -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
