netdev - Re: [PATCH bpf] bpf, x64: implement retpoline for tail call

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Date:   Wed, 21 Feb 2018 20:59:08 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>, ast@...nel.org,
        torvalds@...ux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH bpf] bpf, x64: implement retpoline for tail call

On Wed, Feb 21, 2018 at 07:53:22PM -0800, Eric Dumazet wrote:
> > So what kinda comment there would make sense?
> 
> I was thinking of something very explicit :
> 
> /* byte sequence for following assembly code used by eBPF
>    call ...
>    ...
>    retq
> */
> #define RETPOLINE_RAX_DIRECT_FOR_EBPF                         \
>        EMIT1_off32(0xE8, 7);    /* callq <set_up_target> */   \
>        /* capture_spec: */                                    \
>        EMIT2(0xF3, 0x90);       /* pause */                   \
>        EMIT3(0x0F, 0xAE, 0xE8); /* lfence */                  \
>        EMIT2(0xEB, 0xF9);       /* jmp <capture_spec> */      \
>        /* set_up_target: */                                   \
>        EMIT4(0x48, 0x89, 0x04, 0x24); /* mov %rax,(%rsp) */   \
>        EMIT1(0xC3);             /* retq */                    \

got it. yeah. makes sense to me.