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    powerpc/bpf: Implement support for tail calls · ce076141
    Naveen N. Rao 提交于
    Tail calls allow JIT'ed eBPF programs to call into other JIT'ed eBPF
    programs. This can be achieved either by:
    (1) retaining the stack setup by the first eBPF program and having all
    subsequent eBPF programs re-using it, or,
    (2) by unwinding/tearing down the stack and having each eBPF program
    deal with its own stack as it sees fit.
    
    To ensure that this does not create loops, there is a limit to how many
    tail calls can be done (currently 32). This requires the JIT'ed code to
    maintain a count of the number of tail calls done so far.
    
    Approach (1) is simple, but requires every eBPF program to have (almost)
    the same prologue/epilogue, regardless of whether they need it. This is
    inefficient for small eBPF programs which may not sometimes need a
    prologue at all. As such, to minimize impact of tail call
    implementation, we use approach (2) here which needs each eBPF program
    in the chain to use its own prologue/epilogue. This is not ideal when
    many tail calls are involved and when all the eBPF programs in the chain
    have similar prologue/epilogue. However, the impact is restricted to
    programs that do tail calls. Individual eBPF programs are not affected.
    
    We maintain the tail call count in a fixed location on the stack and
    updated tail call count values are passed in through this. The very
    first eBPF program in a chain sets this up to 0 (the first 2
    instructions). Subsequent tail calls skip the first two eBPF JIT
    instructions to maintain the count. For programs that don't do tail
    calls themselves, the first two instructions are NOPs.
    Signed-off-by: NNaveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
    Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>
    ce076141
ppc-opcode.h 15.8 KB