linux-kernel - Re: [PATCH] LoongArch: BPF: Sign-extend return values

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Message-ID: <4f6c74e0-dd22-8460-96fa-f408291a3ef8@loongson.cn>
Date: Fri, 22 Nov 2024 15:39:49 +0800
From: Tiezhu Yang <yangtiezhu@...ngson.cn>
To: John Fastabend <john.fastabend@...il.com>,
 Huacai Chen <chenhuacai@...nel.org>
Cc: loongarch@...ts.linux.dev, bpf@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] LoongArch: BPF: Sign-extend return values

On 11/22/2024 08:41 AM, John Fastabend wrote:
> Tiezhu Yang wrote:
>> (1) Description of Problem:
>>
>> When testing BPF JIT with the latest compiler toolchains on LoongArch,
>> there exist some strange failed test cases, dmesg shows something like
>> this:
>>
>>   # dmesg -t | grep FAIL | head -1
>>   ... ret -3 != -3 (0xfffffffd != 0xfffffffd)FAIL ...

...

>>
>> (5) Final Solution:
>>
>> Keep a5 zero-extended, but explicitly sign-extend a0 (which is used
>> outside BPF land). Because libbpf currently defines the return value
>> of an ebpf program as a 32-bit unsigned integer, just use addi.w to
>> extend bit 31 into bits 63 through 32 of a5 to a0. This is similar
>> with commit 2f1b0d3d7331 ("riscv, bpf: Sign-extend return values").
>>
>> Fixes: 5dc615520c4d ("LoongArch: Add BPF JIT support")
>> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
>> ---
>>  arch/loongarch/net/bpf_jit.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/loongarch/net/bpf_jit.c b/arch/loongarch/net/bpf_jit.c
>> index 7dbefd4ba210..dd350cba1252 100644
>> --- a/arch/loongarch/net/bpf_jit.c
>> +++ b/arch/loongarch/net/bpf_jit.c
>> @@ -179,7 +179,7 @@ static void __build_epilogue(struct jit_ctx *ctx, bool is_tail_call)
>>
>>  	if (!is_tail_call) {
>>  		/* Set return value */
>> -		move_reg(ctx, LOONGARCH_GPR_A0, regmap[BPF_REG_0]);
>> +		emit_insn(ctx, addiw, LOONGARCH_GPR_A0, regmap[BPF_REG_0], 0);
>
> Not overly familiar with this JIT but just to check this wont be used
> for BPF 2 BPF calls correct?

I am not sure I understand your comment correctly, but with and without
this patch, the LoongArch JIT uses a5 as a dedicated register for BPF
return values, a5 is kept as zero-extended for bpf2bpf, just make a0
(which is used outside BPF land) as sign-extend, all of the test cases
in test_bpf.ko passed with "echo 1 > /proc/sys/net/core/bpf_jit_enable".

Thanks,
Tiezhu