On Wed, Aug 17, 2016 at 04:32:23PM +0200, Dr. Philipp Tomsich wrote: > 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. I don't particularly like splitting 64-bit arguments in two 32-bit values either but I don't see a better alternative. To keep this mostly in the arch code we would need an additional table of syscall wrappers where the majority just use the default zero-extend everything with a few specific wrappers where we pass 64-bit arguments. Or we could set an extra bit in the syscall number for those syscalls that need special wrapping and avoid zero-extending. But neither of these look any nicer (well, maybe only from the user-space perspective). > Everything points to just going with the pair-of-registers and getting > this merged quickly then, I suppose. I will refrain from commenting on how quickly we merge this ;) (it may be seen as binding by some). -- 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
