netdev - Re: [PATCH bpf-next v7 4/4] bpf, arm64: bpf trampoline for arm64

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Date:   Mon, 11 Jul 2022 15:48:13 +0100
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Xu Kuohai <xukuohai@...wei.com>
Cc:     bpf@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        Will Deacon <will@...nel.org>, KP Singh <kpsingh@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Zi Shen Lim <zlim.lnx@...il.com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Russell King <rmk+kernel@...linux.org.uk>,
        James Morse <james.morse@....com>,
        Hou Tao <houtao1@...wei.com>,
        Jason Wang <wangborong@...rlc.com>
Subject: Re: [PATCH bpf-next v7 4/4] bpf, arm64: bpf trampoline for arm64

On Mon, Jul 11, 2022 at 10:40:42PM +0800, Xu Kuohai wrote:
> On 7/11/2022 10:37 PM, Jean-Philippe Brucker wrote:
> > On Mon, Jul 11, 2022 at 10:16:00PM +0800, Xu Kuohai wrote:
> >>>> +	if (save_ret)
> >>>> +		emit(A64_STR64I(p->jited ? r0 : A64_R(0), A64_SP, retval_off),
> >>>> +		     ctx);
> >>>
> >>> This should be only A64_R(0), not r0. r0 happens to equal A64_R(0) when
> >>> jitted due to the way build_epilogue() builds the function at the moment,
> >>> but we shouldn't rely on that.
> >>>
> >>
> >> looks like I misunderstood something, will change it to:
> >>
> >> /* store return value, which is held in x0 for interpreter and in
> >>  * bpf register r0 for JIT,
> > 
> > It's simpler than that: in both cases the return value is in x0 because
> > the function follows the procedure call standard. You could drop the
> > comment to avoid confusion and only do the change to A64_R(0)
> > 
> 
> OK, will send v9 since v8 was just sent

Right sorry about this, I could have been clearer

Thanks,
Jean