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Subject: Re: [RFC PATCH 2/2] powerpc/64s: system call support for scv/rfscv
 instructions
To: Nicholas Piggin <npiggin@gmail.com>, linuxppc-dev@lists.ozlabs.org
References: <20200430040202.1735506-1-npiggin@gmail.com>
 <20200430040202.1735506-3-npiggin@gmail.com>
From: Matheus Castanho <msc@linux.ibm.com>
Message-ID: <cc609f82-1d11-b1f9-2594-153936d7fe48@linux.ibm.com>
Date: Wed, 13 May 2020 17:55:07 -0300
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

Hi Nicholas,

Small comment below:

On 4/30/20 1:02 AM, Nicholas Piggin wrote:
> Add support for the scv instruction on POWER9 and later CPUs.
> 
> For now this implements the zeroth scv vector 'scv 0', as identical
> to 'sc' system calls, with the exception that lr is not preserved, and
> it is 64-bit only. There may yet be changes made to this ABI, so it's
> for testing only.
> 
> rfscv is implemented to return from scv type system calls. It can not
> be used to return from sc system calls because those are defined to
> preserve lr.
> 
> In a comparison of getpid syscall, the test program had scv taking
> about 3 more cycles in user mode (92 vs 89 for sc), due to lr handling.
> getpid syscall throughput on POWER9 is improved by 33%, mostly due to
> reducing mtmsr and mtspr.
> 
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>  Documentation/powerpc/syscall64-abi.rst   |  42 ++++--

[...]

> +Return value
> +------------
> +- For the sc instruction, both a return value and a return error code are
> +  returned. cr0.SO is the return error code, and r3 is the return value or
> +  error code. When cr0.SO is clear, the syscall succeeded and r3 is the return
> +  value. When cr0.SO is set, the syscall failed and r3 is the error code that
> +  generally corresponds to errno.
> +
> +- For the scv 0 instruction, there is a return value indicates failure if it
> +  is >= -MAX_ERRNO (-4095) as an unsigned comparison, in which case it is the
> +  negated return error code. Otherwise it is the successful return value.

I believe this last paragraph is a bit confusing (didn't quite get the
unsigned comparison with negative values). So instead of cr0.SO to
indicate failure, scv returns the negated error code, and positive in
case of success?

Thanks,
Matheus Castanho