Re: [RFC2 nowrap: PATCH v7 00/18] ILP32 for ARM64

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Aug 17, 2016 at 02:54:59PM +0200, Dr. Philipp Tomsich wrote:
> On 17 Aug 2016, at 14:48, Yury Norov <ynorov@caviumnetworks.com> wrote:
> > On Wed, Aug 17, 2016 at 02:28:50PM +0200, Alexander Graf wrote:
> >> On 17 Aug 2016, at 13:46, Yury Norov <ynorov@caviumnetworks.com> 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