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

[Christophe Leroy]

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