Re: [RFC PATCH v1 7/7] powerpc/bpf: Implement extended BPF on PPC32

From: Christophe Leroy <christophe.leroy@csgroup.eu>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org,
	kafai@fb.com, songliubraving@fb.com, yhs@fb.com,
	john.fastabend@gmail.com, kpsingh@chromium.org,
	naveen.n.rao@linux.ibm.com, sandipan@linux.ibm.com,
	linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	netdev@vger.kernel.org, bpf@vger.kernel.org
Subject: Re: [RFC PATCH v1 7/7] powerpc/bpf: Implement extended BPF on PPC32
Date: Thu, 17 Dec 2020 10:54:34 +0100	[thread overview]
Message-ID: <854404a0-1951-91d9-2ebb-208390a64c77@csgroup.eu> (raw)
In-Reply-To: <20201217061133.lnfnhbzvikgtjb3i@ast-mbp>

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