[PATCH 00/23] powerpc: Syscall wrapper and register clearing

[Rohan McLure]

> On 16 Sep 2022, at 3:32 pm, Rohan McLure <rmclure at linux.ibm.com> wrote:
> 
> V4 available here:
> 
> Link: https://lore.kernel.org/all/20220824020548.62625-1-rmclure@linux.ibm.com/
> 
> Implement a syscall wrapper, causing arguments to handlers to be passed
> via a struct pt_regs on the stack. The syscall wrapper is implemented
> for all platforms other than the Cell processor, from which SPUs expect
> the ability to directly call syscall handler symbols with the regular
> in-register calling convention.
> 
> Adopting syscall wrappers requires redefinition of architecture-specific
> syscalls and compatibility syscalls to use the SYSCALL_DEFINE and
> COMPAT_SYSCALL_DEFINE macros, as well as removal of direct-references to
> the emitted syscall-handler symbols from within the kernel. This work
> lead to the following modernisations of powerpc's syscall handlers:
> 
> - Replace syscall 82 semantics with sys_old_select and remove
>   ppc_select handler, which features direct call to both sys_old_select
>   and sys_select.
> - Use a generic fallocate compatibility syscall
> 
> Replace asm implementation of syscall table with C implementation for
> more compile-time checks.
> 
> Many compatibility syscalls are candidates to be removed in favour of
> generically defined handlers, but exhibit different parameter orderings
> and numberings due to 32-bit ABI support for 64-bit parameters. The
> parameter reorderings are however consistent with arm. A future patch
> series will serve to modernise syscalls by providing generic
> implementations featuring these reorderings.
> 
> The design of this syscall is very similar to the s390, x86 and arm64
> implementations. See also Commit 4378a7d4be30 (arm64: implement syscall wrappers).
> The motivation for this change is that it allows for the clearing of
> register state when entering the kernel via through interrupt handlers
> on 64-bit servers. This serves to reduce the influence of values in
> registers carried over from the interrupted process, e.g. syscall
> parameters from user space, or user state at the site of a pagefault.
> All values in registers are saved and zeroized at the entry to an
> interrupt handler and restored afterward. While this may sound like a
> heavy-weight mitigation, many gprs are already saved and restored on
> handling of an interrupt, and the mmap_bench benchmark on Power 9 guest,
> repeatedly invoking the pagefault handler suggests at most ~0.8%
> regression in performance. Realistic workloads are not constantly
> producing interrupts, and so this does not indicate realistic slowdown.
> 
> Using wrapped syscalls yields to a performance improvement of ~5.6% on
> the null_syscall benchmark on pseries guests, by removing the need for
> system_call_exception to allocate its own stack frame. This amortises
> the additional costs of saving and restoring non-volatile registers
> (register clearing is cheap on super scalar platforms), and so the
> final mitigation actually yields a net performance improvement of ~0.6%
> on the null_syscall benchmark.
> 
> The clearing of general purpose registers on interrupts other than
> syscalls is enabled by default only on Book3E 64-bit systems (where the
> mitigation is inexpensive), but available to other 64-bit systems via
> the INTERRUPT_SANITIZE_REGISTERS Kconfig option. This mitigation is
> optional, as the speculation influence of interrupts is likely less than
> that of syscalls.
> 
> Patch Changelog:
> 
> - Format orig_r3 handling as its own patch rather than just a revert.
> - Provide asm-generic BE implementation of long-long munging syscall
>   compatiblity arguments.
> - Syscall #82 now refers to generic sys_old_select or
>   comptat_sys_old_select.
> - Drop 'inline' on static helper functions for mmap, personality.
> - Remove arch-specific sys fallocate implementation that was meant to
>   have been removed in V2.
> - Remove references to syscall wrapper until it is introduced.
> - Rearrange patch series so the last five patches are syscall wrapper >
>   syscall register clears > interrupt register clears.
> - Whether non-syscall interrupts should clear registers is now
>   configurable by INTERRUPT_SANITIZE_REGISTERS.
> 
> Rohan McLure (23):
>  powerpc: Remove asmlinkage from syscall handler definitions
>  powerpc: Save caller r3 prior to system_call_exception
>  powerpc: Add ZEROIZE_GPRS macros for register clears
>  powerpc/64s: Use {ZEROIZE,SAVE,REST}_GPRS macros in sc, scv 0 handlers
>  powerpc/32: Clarify interrupt restores with REST_GPR macro in
>    entry_32.S
>  powerpc/64e: Clarify register saves and clears with
>    {SAVE,ZEROIZE}_GPRS
>  powerpc/64s: Fix comment on interrupt handler prologue
>  powerpc: Fix fallocate and fadvise64_64 compat parameter combination
>  asm-generic: compat: Support BE for long long args in 32-bit ABIs
>  powerpc: Use generic fallocate compatibility syscall
>  powerpc/32: Remove powerpc select specialisation
>  powerpc: Remove direct call to personality syscall handler
>  powerpc: Remove direct call to mmap2 syscall handlers
>  powerpc: Provide do_ppc64_personality helper
>  powerpc: Adopt SYSCALL_DEFINE for arch-specific syscall handlers
>  powerpc: Include all arch-specific syscall prototypes
>  powerpc: Enable compile-time check for syscall handlers
>  powerpc: Use common syscall handler type
>  powerpc: Provide syscall wrapper
>  powerpc/64s: Clear/restore caller gprs in syscall interrupt/return
>  powerpc/64: Add INTERRUPT_SANITIZE_REGISTERS Kconfig
>  powerpc/64s: Clear gprs on interrupt routine entry in Book3S
>  powerpc/64e: Clear gprs on interrupt routine entry on Book3E
> 
> arch/powerpc/Kconfig                         |  10 ++
> arch/powerpc/include/asm/interrupt.h         |   3 +-
> arch/powerpc/include/asm/ppc_asm.h           |  22 +++
> arch/powerpc/include/asm/syscall.h           |  11 +-
> arch/powerpc/include/asm/syscall_wrapper.h   |  84 ++++++++++
> arch/powerpc/include/asm/syscalls.h          | 148 +++++++++++++----
> .../ppc32.h => include/asm/syscalls_32.h}    |   0
> arch/powerpc/include/asm/unistd.h            |   1 +
> arch/powerpc/kernel/entry_32.S               |  40 ++---
> arch/powerpc/kernel/exceptions-64e.S         |  35 ++--
> arch/powerpc/kernel/exceptions-64s.S         |  41 ++++-
> arch/powerpc/kernel/interrupt_64.S           |  92 +++++-----
> arch/powerpc/kernel/signal_32.c              |   2 +-
> arch/powerpc/kernel/sys_ppc32.c              |  66 +++-----
> arch/powerpc/kernel/syscall.c                |  32 ++--
> arch/powerpc/kernel/syscalls.c               |  61 ++++---
> arch/powerpc/kernel/syscalls/syscall.tbl     |  24 +--
> arch/powerpc/kernel/{systbl.S => systbl.c}   |  30 ++--
> arch/powerpc/kernel/vdso.c                   |   6 +-
> arch/powerpc/perf/callchain_32.c             |   2 +-
> arch/powerpc/platforms/cell/spu_callbacks.c  |   6 +-
> include/asm-generic/compat.h                 |   9 +-
> .../arch/powerpc/entry/syscalls/syscall.tbl  |  24 +--
> 23 files changed, 491 insertions(+), 258 deletions(-)
> create mode 100644 arch/powerpc/include/asm/syscall_wrapper.h
> rename arch/powerpc/{kernel/ppc32.h => include/asm/syscalls_32.h} (100%)
> rename arch/powerpc/kernel/{systbl.S => systbl.c} (55%)
> 
> -- 
> 2.34.1
> 

Apologies. This is in fact V5. I can resend if anyone needs me to.

Rohan