Re: [PATCH v6 21/25] powerpc: Provide syscall wrapper

From: "Nicholas Piggin" <npiggin@gmail.com>
To: "Rohan McLure" <rmclure@linux.ibm.com>, <linuxppc-dev@lists.ozlabs.org>
Cc: Andrew Donnellan <ajd@linux.ibm.com>
Subject: Re: [PATCH v6 21/25] powerpc: Provide syscall wrapper
Date: Fri, 23 Sep 2022 17:50:29 +1000	[thread overview]
Message-ID: <CN3M7LAXFI0O.J739TZ6ZPHN0@bobo> (raw)
In-Reply-To: <20220921065605.1051927-22-rmclure@linux.ibm.com>

On Wed Sep 21, 2022 at 4:56 PM AEST, Rohan McLure wrote:
> Implement syscall wrapper as per s390, x86, arm64. When enabled
> cause handlers to accept parameters from a stack frame rather than
> from user scratch register state. This allows for user registers to be
> safely cleared in order to reduce caller influence on speculation
> within syscall routine. The wrapper is a macro that emits syscall
> handler symbols that call into the target handler, obtaining its
> parameters from a struct pt_regs on the stack.
>
> As registers are already saved to the stack prior to calling
> system_call_exception, it appears that this function is executed more
> efficiently with the new stack-pointer convention than with parameters
> passed by registers, avoiding the allocation of a stack frame for this
> method. On a 32-bit system, we see >20% performance increases on the
> null_syscall microbenchmark, and on a Power 8 the performance gains
> amortise the cost of clearing and restoring registers which is
> implemented at the end of this series, seeing final result of ~5.6%
> performance improvement on null_syscall.
>
> Syscalls are wrapped in this fashion on all platforms except for the
> Cell processor as this commit does not provide SPU support. This can be
> quickly fixed in a successive patch, but requires spu_sys_callback to
> allocate a pt_regs structure to satisfy the wrapped calling convention.
>
> Co-developed-by: Andrew Donnellan <ajd@linux.ibm.com>
> Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
> Signed-off-by: Rohan McLure <rmclure@linux.ibm.com>
> ---
> V2: Generate prototypes for symbols produced by the wrapper.
> V3: Rebased to remove conflict with 1547db7d1f44
> ("powerpc: Move system_call_exception() to syscall.c"). Also remove copy
> from gpr3 save slot on stackframe to orig_r3's slot. Fix whitespace with
> preprocessor defines in system_call_exception.
> V5: Move systbl.c syscall wrapper support to this patch. Swap
> calling convention for system_call_exception to be (&regs, r0)
> V6: Change calling convention for system_call_exception in another
> patch.
> ---

Nice. Looks very clean.

> diff --git a/arch/powerpc/kernel/vdso.c b/arch/powerpc/kernel/vdso.c
> index fcca06d200d3..e1f36fd61db3 100644
> --- a/arch/powerpc/kernel/vdso.c
> +++ b/arch/powerpc/kernel/vdso.c
> @@ -39,6 +39,8 @@
>  extern char vdso32_start, vdso32_end;
>  extern char vdso64_start, vdso64_end;
>  
> +long sys_ni_syscall(void);
> +
>  /*
>   * The vdso data page (aka. systemcfg for old ppc64 fans) is here.
>   * Once the early boot kernel code no longer needs to muck around

What's this doing? Why can't it continue using the declaration in
syscalls.h?

Aside from that and the stray hunk to go in the previous patch,

Reviewed-by: Nicholas Piggin <npiggin@gmai.com>

Thanks,
Nick