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

From: Xu Kuohai <xukuohai@huawei.com>
To: Jean-Philippe Brucker <jean-philippe@linaro.org>
Cc: <bpf@vger.kernel.org>, <linux-arm-kernel@lists.infradead.org>,
	<linux-kernel@vger.kernel.org>, <netdev@vger.kernel.org>,
	Will Deacon <will@kernel.org>, KP Singh <kpsingh@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Alexei Starovoitov <ast@kernel.org>,
	Andrii Nakryiko <andrii@kernel.org>,
	Zi Shen Lim <zlim.lnx@gmail.com>, Martin KaFai Lau <kafai@fb.com>,
	Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	"David S . Miller" <davem@davemloft.net>,
	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
	David Ahern <dsahern@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>, <x86@kernel.org>,
	"H . Peter Anvin" <hpa@zytor.com>,
	Jakub Kicinski <kuba@kernel.org>,
	Jesper Dangaard Brouer <hawk@kernel.org>,
	Russell King <rmk+kernel@armlinux.org.uk>,
	James Morse <james.morse@arm.com>, Hou Tao <houtao1@huawei.com>,
	Jason Wang <wangborong@cdjrlc.com>
Subject: Re: [PATCH bpf-next v7 4/4] bpf, arm64: bpf trampoline for arm64
Date: Mon, 11 Jul 2022 22:40:42 +0800	[thread overview]
Message-ID: <893b2d5f-16e9-0b1d-4ae6-8199e0f4ccf8@huawei.com> (raw)
In-Reply-To: <Ysw1nSxQUghvY/eY@myrica>

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

> Thanks,
> Jean
> 
>>
>>
>>  but r0 happens to equal x0 due to the
>>  * way build_epilogue() builds the JIT image.
>>  */
>> if (save_ret)
>>         emit(A64_STR64I(A64_R(0), A64_SP, retval_off), ctx);
>>
>>> Apart from that, for the series
>>>
>>> Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
>>>
>>> .
> .


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