Sub chain cross technology source level exploration: an overview of xcvm

Continue with an article on Subtrate Preliminary exploration of several element components across the chain ,  Let's decode this article XCVM,  Before that ,  We need to know first XCVM Execution is XCM news ,  So let's review it a little first XCM Format of message .

Premise : XCM Message format and version

XCM From the beginning of development to the present ,  It's near 2 More than a year ,  Period version from v0 Up to now, it has been officially launched V2,  And what is being improved V3,  Experienced many changes ,  There is a cumulative superposition between versions ,  And the process of substantial changes .

Especially in V1 To V2 In the process of ,  The evolution of its source code is quite fierce ,  In the previous post ,  We see : 

stay  V0  and  V1  In the version , Xcm  It's a man named

Xcm<Call>

Of

enum

,  In order to facilitate reading comprehension ,  The code is simplified here :

#[scale_info(bounds(), skip_type_params(Call))]pub enum Xcm<Call> { //...snippets WithdrawAsset(MultiAssets), ReserveAssetDeposited(MultiAssets), ReceiveTeleportedAsset(MultiAssets), //...snippets}


And in the  V2  and  V3  In the version , Xcm  It was replaced by a

Tuple Struct

Tuple structure :

#[scale_info(bounds(), skip_type_params(Call))]pub struct Xcm<Call>(pub Vec<Instruction<Call>>);


If we look carefully ,  Will find  V2  Version later  Xcm  The tuple structure is actually a  Vec  aggregate ,  Its aggregated child element is a named

Instruction<Call>

Enumeration .

therefore ,  If the XCM when ,  The version must be marked ,  The version identification is as follows :

• version: u8

• message
  : the message,  Must be in version Valid in certain cases

• Version: u8 = 2
  , 
  message: Vec<Instruction>
  ,  therefore ,  A complete XCM The message is 2 byte Version ID of + 
  SCALE
  Coded
  XCVM
  Command sequence

• One XCM The final implementation effect ,  It depends on two  
  XCVM
    Whether the two registers are properly initialized : 
  Original Programme
    A message   and  
  Original Origin
  Indicates the source of the information

XCVM register

XCVM As an executive XCM Virtual machine of message instruction ,  In essence, it is a non Turing complete virtual machine with register architecture .

So we need to take a look at

XCVM

Register design ;  As XCM Message format executor , XCVM The register of has a strong constraint ,  It cannot be set at will ,  Instead, you should place specific related values ,  And it can only be changed under specific scenarios and specific rules .

The following are the basic functions and meanings of each register :

Programme

 

Storage type : 

Vec<Instruction>

 

Initial value : 

Original Programme

Value  

effect :  What is currently being implemented

XCVM

Instructions ,  It can only be changed when all instructions are executed or errors occur

 

Programme Counter

Storage type : u32

 

Initial value :0

 

effect :  The instruction currently being executed is

Programme

Index number in register , Every time an instruction is successfully executed +1, 

Programme

The register is reset to when it is updated 0

Error

 

Storage type : Option<(u32, Error)>

 

Initial value : None

 

effect :  Latest instruction program execution error ,  Set only when the program has an error ,  You can take the initiative to clean up . Two of the child values are when the error occurs

Programme Counter

Values and error types  

Error Handler

 

Vec<Instruction>

  An empty list   If an error occurs ,  Instructions that need to be executed ,  When a program error occurs ,  Clear this register ,  And put its value in

Programme

In the implementation of  

Appendix

 

Storage type : Vec<Instruction>

 

Initial value :  An empty list  

effect :  Additional executed instructions ,  When the program is finished ,  Or after an error

Error Handler

After being emptied ,  It will be worth to

Programme

And clear this register  

Origin

 

Storage type : Option<MultiLocation>

 

Initial value :  Internal value set to Original Origin 

effect :  The authorized address of the current program ,  It can be for

None

( No authorization ),  Or strictly internal address ( Limited authorization subset )

Holding Register

 

Storage type : MultiAssets

 

Initial value :  An empty set ( No resources ) 

effect :  Resources controlled by the current program but not linked ,  It can be used as an unused resource (

unspent

) Of

non-persistent

register ,  Temporary storage area for similar resources  

Surplus Weight

 

Storage type : u64

 

Initial value :0

 

effect :  Excess of the current program estimate ,  Include the weight of instructions that are not executed ,  No valid error handling weights ,  And conservative estimated weights for successful execution of instructions  

Refunded Weight

 

Storage type : u64

 

Initial value : 0

 

effect : Surplus Weight

The returned part of , 

XCM

The platform has no execution cost, so this register is not used

XCVM

Basic execution process

XCVM

It's a

fetch-dispatch

" Grab - distribution "  State machine for loop execution ,  The steps are as follows :

• Programme
  When the register is empty ,  downtime

• from
  Programme
  Get in the
  Programme Counter
  The instruction that the index value points to

• If the instruction exists ,  Then execute the instruction ,  And check the execution results :

• If there are no mistakes ,  On behalf of successful execution ,  Will
  Programme Counter
  Add 1

• If an error occurs ,  be

• Set up
  Error
  Register value ( The index where the error occurred and the type of error )

• take
  Programme Counter
  Set as
  0

• take
  Programme
  Replace the content of with
  Error Handler
  The content of ,  And empty  
  Error Handler

• If the instruction doesn't exist ,  be :

• Programme Counter
  Set as
  0

• take
  Programme
  Replace the content of with
  Appendix Register
  The content of

• take
  Error Handler
  The estimated weight of the content in ,  Add to
  Surplus Weight
  In the middle

• Empty  
  Appendix Register
  and
  Error Handler

• With the ordinary
  fetch/dispatch
  The mechanism is different , 
  XCVM
  During the implementation of
  Error Handler
  and
  Appendix
  register ,  It should be noted that :

• Error
  The register will not be cleared after the program is executed ,  This will allow
  Appendix
  Registers can use
  Error
  Values reserved in

• Whether the program is successfully executed or not , 
  Error Handler
  All registers will be cleared ,  This ensures that the error handling logic of the previous program will not affect the subsequent code execution

Be careful

• Instruction type
  Instruction
    Is a label union type (Rust Medium
  enum
  ),  Each instruction consists of
  SCALE
  Encoded in
  0
  Is the position in the instruction list of the initial index + Its operation is built ,  We need to pay attention to

• Origin
  The term is here ,  Express
  Origin
  The position represented by the value in the register ,  therefore  
  controlled by the Origin
  It means the same as " from
  Origin
  The value in the register represents the address control

• Holding
  The term means
  Holding
  Value in register ,  therefore
  reduce Holding by the asset
  be equal to " from
  Holding
  Reduce assets in the register

• Appendix
  and
  Error Handler
  It's similar

• on-chain
  The term ,  It should be understood as " Persistent state under local consensus system (within the persistent state of the local consensus system)",  We should not only treat blockchain as a local consensus .

• Compact
  , 
  Weight
  and
  QueryId
  Type should be understood as
  SCALE
  Packed integer type defined in

• 
  

About XCVM That's all for the register and instruction execution flow of ,  In the next article, we will explore the specific XCM Meaning and use of instructions .