> On 17 Aug 2016, at 16:29, Catalin Marinas <catalin.marinas@xxxxxxx> wrote: > > 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). Yes, I remember the discussions and just wanted to put option #2 in context again. Everything points to just going with the pair-of-registers and getting this merged quickly then, I suppose. Cheers, Philipp.-- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
