linux-kernel - Re: [PATCH bpf-next v2 3/8] arm32, bpf: add support for sign-extension mov instruction

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Message-ID: <ZPmNjOLqC1cAMgGL@shell.armlinux.org.uk>
Date:   Thu, 7 Sep 2023 09:45:00 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Puranjay Mohan <puranjay12@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>,
        Yonghong Song <yonghong.song@...ux.dev>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Shubham Bansal <illusionist.neo@...il.com>,
        Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
        bpf@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 3/8] arm32, bpf: add support for
 sign-extension mov instruction

On Wed, Sep 06, 2023 at 06:33:15PM +0000, Puranjay Mohan wrote:
> The cpuv4 added a new BPF_MOVSX instruction that sign extends the src
> before moving it to the destination.
> 
> BPF_ALU | BPF_MOVSX sign extends 8-bit and 16-bit operands into 32-bit
> operands, and zeroes the remaining upper 32 bits.
> 
> BPF_ALU64 | BPF_MOVSX sign extends 8-bit, 16-bit, and 32-bit  operands
> into 64-bit operands.
> 
> The offset field of the instruction is used to tell the number of bit to
> use for sign-extension. BPF_MOV and BPF_MOVSX have the same code but the
> former sets offset to 0 and the later one sets the offset to 8, 16 or 32
> 
> The behaviour of this instruction is dst = (s8,s16,s32)src
> 
> On ARM32 the implementation uses LSH and ARSH to extend the 8/16 bits to
> a 32-bit register and then it is sign extended to the upper 32-bit
> register using ARSH. For 32-bit we just move it to the destination
> register and use ARSH to extend it to the upper 32-bit register.
> 
> Signed-off-by: Puranjay Mohan <puranjay12@...il.com>

Reviewed-by: Russell King (Oracle) <rmk+kernel@...linux.org.uk>

Thanks!

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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