Le samedi 25 juin 2011 à 09:33 +0200, Andreas Schwab a écrit :
> Matt Evans <m...@ozlabs.org> writes:
>
> > + stdu r1, -128(r1); \
>
> > + addi r5, r1, 128+BPF_PPC_STACK_BASIC+(2*8); \
>
> > + addi r1, r1, 128; \
>
> > + PPC_STD(r_M + i, 1, -128 + (8*i));
>
> > + PPC_LD(r_M + i, 1, -128 + (8*i));
>
> s/128/BPF_PPC_STACK_SAVE/?
>
I am not sure using registers to hold MEM[] is a win if MEM[idx] is used
once in the filter
# tcpdump "tcp[20]+tcp[21]=0" -d
(000) ldh [12]
(001) jeq #0x800 jt 2 jf 15
(002) ldb [23]
(003) jeq #0x6 jt 4 jf 15
(004) ldh [20]
(005) jset #0x1fff jt 15 jf 6
(006) ldxb 4*([14]&0xf)
(007) ldb [x + 34]
(008) st M[1]
(009) ldb [x + 35]
(010) tax
(011) ld M[1]
(012) add x
(013) jeq #0x0 jt 14 jf 15
(014) ret #65535
(015) ret #0
In this sample, we use M[1] once ( one store, one load)
So saving previous register content on stack in prologue, and restoring
it in epilogue actually slow down the code, and adds two instructions in filter
asm code.
This also makes epilogue code not easy (not possible as a matter of fact)
to unwind in helper function
In x86_64 implementation, I chose bpf_error be able to force
an exception, not returning to JIT code but directly to bpf_func() caller
bpf_error:
# force a return 0 from jit handler
xor %eax,%eax
mov -8(%rbp),%rbx
leaveq
ret
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