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Date:   Thu, 17 Dec 2020 10:54:34 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, ast@...nel.org,
        daniel@...earbox.net, andrii@...nel.org, kafai@...com,
        songliubraving@...com, yhs@...com, john.fastabend@...il.com,
        kpsingh@...omium.org, naveen.n.rao@...ux.ibm.com,
        sandipan@...ux.ibm.com, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [RFC PATCH v1 7/7] powerpc/bpf: Implement extended BPF on PPC32



Le 17/12/2020 à 07:11, Alexei Starovoitov a écrit :
> On Wed, Dec 16, 2020 at 10:07:37AM +0000, Christophe Leroy wrote:
>> Implement Extended Berkeley Packet Filter on Powerpc 32
>>
>> Test result with test_bpf module:
>>
>> 	test_bpf: Summary: 378 PASSED, 0 FAILED, [354/366 JIT'ed]
> 
> nice!
> 
>> Registers mapping:
>>
>> 	[BPF_REG_0] = r11-r12
>> 	/* function arguments */
>> 	[BPF_REG_1] = r3-r4
>> 	[BPF_REG_2] = r5-r6
>> 	[BPF_REG_3] = r7-r8
>> 	[BPF_REG_4] = r9-r10
>> 	[BPF_REG_5] = r21-r22 (Args 9 and 10 come in via the stack)
>> 	/* non volatile registers */
>> 	[BPF_REG_6] = r23-r24
>> 	[BPF_REG_7] = r25-r26
>> 	[BPF_REG_8] = r27-r28
>> 	[BPF_REG_9] = r29-r30
>> 	/* frame pointer aka BPF_REG_10 */
>> 	[BPF_REG_FP] = r31
>> 	/* eBPF jit internal registers */
>> 	[BPF_REG_AX] = r19-r20
>> 	[TMP_REG] = r18
>>
>> As PPC32 doesn't have a redzone in the stack,
>> use r17 as tail call counter.
>>
>> r0 is used as temporary register as much as possible. It is referenced
>> directly in the code in order to avoid misuse of it, because some
>> instructions interpret it as value 0 instead of register r0
>> (ex: addi, addis, stw, lwz, ...)
>>
>> The following operations are not implemented:
>>
>> 		case BPF_ALU64 | BPF_DIV | BPF_X: /* dst /= src */
>> 		case BPF_ALU64 | BPF_MOD | BPF_X: /* dst %= src */
>> 		case BPF_STX | BPF_XADD | BPF_DW: /* *(u64 *)(dst + off) += src */
>>
>> The following operations are only implemented for power of two constants:
>>
>> 		case BPF_ALU64 | BPF_MOD | BPF_K: /* dst %= imm */
>> 		case BPF_ALU64 | BPF_DIV | BPF_K: /* dst /= imm */
> 
> Those are sensible limitations. MOD and DIV are rare, but XADD is common.
> Please consider doing it as a cmpxchg loop in the future.
> 
> Also please run test_progs. It will give a lot better coverage than test_bpf.ko
> 

I'm having hard time cross building test_progs:

~/linux-powerpc/tools/testing/selftests/bpf/$ make CROSS_COMPILE=ppc-linux-
...
   GEN 
/home/chr/linux-powerpc/tools/testing/selftests/bpf/tools/build/bpftool/Documentation/bpf-helpers.7
   INSTALL  eBPF_helpers-manpage
   INSTALL  Documentation-man
   GEN      vmlinux.h
/bin/sh: /home/chr/linux-powerpc/tools/testing/selftests/bpf/tools/sbin/bpftool: cannot execute 
binary file
make: *** [/home/chr/linux-powerpc/tools/testing/selftests/bpf/tools/include/vmlinux.h] Error 126
make: *** Deleting file `/home/chr/linux-powerpc/tools/testing/selftests/bpf/tools/include/vmlinux.h'

Looks like it builds bpftool for powerpc and tries to run it on my x86.
How should I proceed ?

Thanks
Christophe

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