> 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. Cheers, Philipp.-- 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
