Re: [PATCH v4 00/20] powerpc: Syscall wrapper and register clearing

[Rohan McLure <rmclure@linux.ibm.com>]

Any comments for this revision? Hopefully these revisions address 32-bit and embedded systems appropriately.

Thanks,
Rohan

> On 24 Aug 2022, at 12:05 pm, Rohan McLure <rmclure@linux.ibm.com> wrote:
> 
> V3 available here:
> 
> Link: https://lore.kernel.org/all/4C3A8815-67FF-41EB-A703-981920CA1201@linux.ibm.com/T/
> 
> 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.
> 
> Patch Changelog:
> 
> - Fix instances where NULLIFY_GPRS were still present
> - Minimise unrecoverable windows in entry_32.S between SRR0/1 restores
>   and RFI
> - Remove all references to syscall symbols prior to introducing syscall
>   wrapper.
> - Remove unnecessary duplication of syscall handlers with sys_... and
>   powerpc_sys_... symbols.
> - Clear non-volatile registers on Book3E systems, as some of these
>   systems feature hardware speculation, and we already unconditionally
>   restore NVGPRS.
> 
> Rohan McLure (20):
>  powerpc: Remove asmlinkage from syscall handler definitions
>  powerpc: Use generic fallocate compatibility syscall
>  powerpc/32: Remove powerpc select specialisation
>  powerpc: Provide do_ppc64_personality helper
>  powerpc: Remove direct call to personality syscall handler
>  powerpc: Remove direct call to mmap2 syscall handlers
>  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: Add ZEROIZE_GPRS macros for register clears
>  Revert "powerpc/syscall: Save r3 in regs->orig_r3"
>  powerpc: Provide syscall wrapper
>  powerpc/64s: Clear/restore caller gprs in syscall interrupt/return
>  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/64s: Clear gprs on interrupt routine entry in Book3S
>  powerpc/64e: Clear gprs on interrupt routine entry
> 
> arch/powerpc/Kconfig                         |   1 +
> arch/powerpc/include/asm/compat.h            |   5 +
> 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          | 128 +++++++++++++----
> .../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         |  31 ++--
> arch/powerpc/kernel/exceptions-64s.S         |  25 ++--
> arch/powerpc/kernel/interrupt_64.S           |  92 +++++-------
> arch/powerpc/kernel/signal_32.c              |   2 +-
> arch/powerpc/kernel/sys_ppc32.c              |  54 ++++---
> arch/powerpc/kernel/syscall.c                |  32 ++---
> arch/powerpc/kernel/syscalls.c               |  51 ++++---
> arch/powerpc/kernel/syscalls/syscall.tbl     |  24 ++--
> arch/powerpc/kernel/{systbl.S => systbl.c}   |  29 ++--
> arch/powerpc/kernel/vdso.c                   |   6 +-
> arch/powerpc/perf/callchain_32.c             |   2 +-
> arch/powerpc/platforms/cell/spu_callbacks.c  |   6 +-
> .../arch/powerpc/entry/syscalls/syscall.tbl  |  24 ++--
> 23 files changed, 415 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
>