appliedzkp的zkevm（4）Call context

1. 引言

曾用名为call_id，即将重命名为call_text。 call_context用于：告知EVM proof，which is the correct context the memory, storage and stack operations should be applied inside，如：

• 1）During a call， stack和memory context变了，而storage context保持未变。
• 2）During a call_data_copy，会根据calling contract 调整the source memory context。
• 3）During a return_data_copy，会根据最后返回的call_context调整the source memory context。（This is not tracked inside the call_context but is good to note）

call_context用于track the correct call context that the memory, storage and stack ops are applied to。

2. call_context内容

call_context中包含的元素有：

struct CallContext {
    msg_sender: EthAddress
    gas_available: u256
    gas_used_before_this_call: u256
    // tells us if this transaction will throw or revert in the future 
    is_persistant: bool
    is_static_call: bool
    // a count of the number of storage updates that need to be undone. 
    revert_todo: u256
    // This is the global counter minimum global counter that needs to be reverted.
    gc_to_revert_to: u256
    call_depth: u256
}

impl Commitable for CallContext {
    fn commit(){
        return (
            msg_sender + 
            gas_available * r + 
            gas_used_before_this_call * r**2 +
            is_persistant * r**3 +
            is_static_call * r**4 +
            revert_todo * r**5 +
            gc_to_revert_to * r**6 +
            call_depth * r**7
        )
    }
}

即call_context为a random linear combination of：

• 1）msg.sender
• 2）gas_available
• 3）gas_used_before_this_call
• 4）is_persistant：为binary flag，用于标记该交易未来是否将revert或throw。
• 5）is_static_call
• 6）revert_todo：为计数器，表示undone对应所需的storage updates数。
• 7）gc_to_revert_to：为revert所需的最小global counter。
• 8）call_depth

其中 r = h a s h ( m s g . s e n d e r . . . c a l l _ d e p t h ) r=hash(msg.sender...call\_depth) r=hash(msg.sender...call_depth)，相应的call_context commitment值表示为： C o m ( c a l l _ c o n t e x t ) = m s g . s e n d e r + g a s _ a v a i l a b l e ∗ r . . . + c a l l _ d e p t h ∗ r 7 Com(call\_context)=msg.sender+gas\_available * r ...+ call\_depth*r^7 Com(call_context)=msg.sender+gas_available∗r...+call_depth∗r7

3. 其它相关条款 3.1 Reverts/throws

若Reverts/throws，需做如下动作：

• 1）Find how far back we need to revert：
• 2）revert all storage operations until that point
• 3）若为REVERT op，其为一种特殊的错误场景，除以上2个动作外，还需额外refund left gas。而其它错误场景会consume all left gas。

3.2 Storage

当is_persist = 1时，正常进行storage updates操作即可。 当is_persist = 0时，进行storage updates操作时，需做好revert every storage we do的准备。需要对每次storage update进行计数。我们根据职能合约中的定义执行所有storage updates。当遇到revert操作时，需undo all these updates。

通常会强制Prover展示在gc和gc_to_revert_to之间发生的todo_revert storage writes。 为了简化，在storage write bus mapping中会包含 写入前的值 和 写入后的值。当revert时，当gc