On 17 Aug 2016, at 14:48, Yury Norov [off-list ref] wrote:
On Wed, Aug 17, 2016 at 02:28:50PM +0200, Alexander Graf wrote:
quoted
quoted
On 17 Aug 2016, at 13:46, Yury Norov [off-list ref] 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.