authors: Jason Lowe-Power
last edited: 2023-01-11 16:05:55 +0000

Let’s start on our first state machine file! First, we will create the L1 cache controller for our MSI protocol.

Create a file called MSI-cache.sm and the following code declares the state machine.

machine(MachineType:L1Cache, "MSI cache")
    : <parameters>
{
    <All state machine code>
}

The first thing you’ll notice about the state machine code is that is looks very C++-like. The state machine file is like creating a C++ object in a header file, if you included all of the code there as well. When in doubt, C++ syntax with probably work in SLICC. However, there are many cases where C++ syntax is incorrect syntax for SLICC as well as cases where SLICC extends the syntax.

With MachineType:L1Cache, we are naming this state machine L1Cache. SLICC will generate many different objects for us from the state machine using that name. For instance, once this file is compiled, there will be a new SimObject: L1Cache_Controller that is the cache controller. Also included in this declaration is a description of this state machine: “MSI cache”.

There are many cases in SLICC where you must include a description to go along with the variable. The reason for this is that SLICC was originally designed to just describe, not implement, coherence protocols. Today, these extra descriptions serve two purposes. First, they act as comments on what the author intended each variable, or state, or event, to be used for. Second, many of them are still exported into HTML when building the HTML tables for the SLICC protocol. Thus, while browsing the HTML table, you can see the more detailed comments from the author of the protocol. It is important to be clear with these descriptions since coherence protocols can get quite complicated.

State machine parameters

Proceeding the machine() declaration is a colon, after which all of the parameters to the state machine are declared. These parameters are directly exported to the SimObject that is generated by the state machine.

For our MSI L1 cache, we have the following parameters:

machine(MachineType:L1Cache, "MSI cache")
: Sequencer *sequencer;
  CacheMemory *cacheMemory;
  bool send_evictions;

  <Message buffer declarations>

  {

  }

First, we have a Sequencer. This is a special class that is implemented in Ruby to interface with the rest of gem5. The Sequencer is a gem5 MemObject with a slave port so it can accept memory requests from other objects. The sequencer accepts requests from a CPU (or other master port) and converts the gem5 the packet into a RubyRequest. Finally, the RubyRequest is pushed onto the mandatoryQueue of the state machine. We will revisit the mandatoryQueue in the in-port section.

Next, there is a CacheMemory object. This is what holds the cache data (i.e., cache entries). The exact implementation, size, etc. is configurable at runtime.

Finally, we can specify any other parameters we would like, similar to a general SimObject. In this case, we have a boolean variable send_evictions. This is used for out-of-order core models to notify the load-store queue if an address is evicted after a load to squash a load if it is speculative.

Next, also in the parameter block (i.e., before the first open bracket), we need to declare all of the message buffers that this state machine will use. Message buffers are the interface between the state machine and the Ruby network. Messages are sent and received via the message buffers. Thus, for each virtual channel in our protocol we need a separate message buffer.

The MSI protocol needs three different virtual networks. Virtual networks are needed to prevent deadlock (e.g., it is bad if a response gets stuck behind a stalled request). In this protocol, the highest priority is responses (virtual network 2), followed by forwarded requests (virtual network 1), then requests have the lowest priority (virtual network 0). See Sorin et al. for details on why these three virtual networks are needed.

The following code declares all of the needed message buffers.

machine(MachineType:L1Cache, "MSI cache")
: Sequencer *sequencer;
  CacheMemory *cacheMemory;
  bool send_evictions;

  MessageBuffer * requestToDir, network="To", virtual_network="0", vnet_type="request";
  MessageBuffer * responseToDirOrSibling, network="To", virtual_network="2", vnet_type="response";

  MessageBuffer * forwardFromDir, network="From", virtual_network="1", vnet_type="forward";
  MessageBuffer * responseFromDirOrSibling, network="From", virtual_network="2", vnet_type="response";

  MessageBuffer * mandatoryQueue;

{

}

We have five different message buffers: two “To”, two “From”, and one special message buffer. The “To” message buffers are similar to slave ports in gem5. These are the message buffers that this controller uses to send messages to other controllers in the system. The “From” message buffers are like slave ports. This controller receives messages on “From” buffers from other controllers in the system.

We have two different “To” buffers, one for low priority requests, and one for high priority responses. The priority for the networks are not inherent. The priority is based on the order that other controllers look at the message buffers. It is a good idea to number the virtual networks so that higher numbers mean higher priority, but the virtual network number is ignored by Ruby except that messages on network 2 can only go to other message buffers on network 2 (i.e., messages can’t jump from one network to another).

Similarly, there is two different ways this cache can receive messages, either as a forwarded request from the directory (e.g., another cache requests a writable block and we have a readable copy) or as a response to a request this controller made. The response is higher priority than the forwarded requests.

Finally, there is a special message buffer, the mandatoryQueue. This message buffer is used by the Sequencer to convert gem5 packets into Ruby requests. Unlike the other message buffers, mandatoryQueue does not connect to the Ruby network. Note: the name of this message buffer is hard-coded and must be exactly “mandatoryQueue”.

As previously mentioned, this parameter block is converted into the SimObject description file. Any parameters you put in this block will be SimObject parameters that are accessible from the Python configuration files. If you look at the generated file L1Cache_Controller.py, it will look very familiar. Note: This is a generated file and you should never modify generated files directly!

from m5.params import *
from m5.SimObject import SimObject
from Controller import RubyController

class L1Cache_Controller(RubyController):
    type = 'L1Cache_Controller'
    cxx_header = 'mem/protocol/L1Cache_Controller.hh'
    sequencer = Param.RubySequencer("")
    cacheMemory = Param.RubyCache("")
    send_evictions = Param.Bool("")
    requestToDir = Param.MessageBuffer("")
    responseToDirOrSibling = Param.MessageBuffer("")
    forwardFromDir = Param.MessageBuffer("")
    responseFromDirOrSibling = Param.MessageBuffer("")
    mandatoryQueue = Param.MessageBuffer("")

State declarations

The next part of the state machine is the state declaration. Here, we are going to declare all of the stable and transient states for the state machine. We will follow the naming convention in Sorin et al. For instance, the transient state “IM_AD” corresponds to moving from Invalid to Modified waiting on acks and data. These states come directly from the left column of Table 8.3 in Sorin et al.

state_declaration(State, desc="Cache states") {
    I,      AccessPermission:Invalid,
                desc="Not present/Invalid";

    // States moving out of I
    IS_D,   AccessPermission:Invalid,
                desc="Invalid, moving to S, waiting for data";
    IM_AD,  AccessPermission:Invalid,
                desc="Invalid, moving to M, waiting for acks and data";
    IM_A,   AccessPermission:Busy,
                desc="Invalid, moving to M, waiting for acks";

    S,      AccessPermission:Read_Only,
                desc="Shared. Read-only, other caches may have the block";

    // States moving out of S
    SM_AD,  AccessPermission:Read_Only,
                desc="Shared, moving to M, waiting for acks and 'data'";
    SM_A,   AccessPermission:Read_Only,
                desc="Shared, moving to M, waiting for acks";

    M,      AccessPermission:Read_Write,
                desc="Modified. Read & write permissions. Owner of block";

    // States moving to Invalid
    MI_A,   AccessPermission:Busy,
                desc="Was modified, moving to I, waiting for put ack";
    SI_A,   AccessPermission:Busy,
                desc="Was shared, moving to I, waiting for put ack";
    II_A,   AccessPermission:Invalid,
                desc="Sent valid data before receiving put ack. "Waiting for put ack.";
}

Each state has an associated access permission: “Invalid”, “NotPresent”, “Busy”, “Read_Only”, or “Read_Write”. The access permission is used for functional accesses to the cache. Functional accesses are debug-like accesses when the simulator wants to read or update the data immediately. One example of this is reading in files in SE mode which are directly loaded into memory.

For functional accesses all caches are checked to see if they have a corresponding block with matching address. For functional reads, all of the blocks with a matching address that have read-only or read-write permission are accessed (they should all have the same data). For functional writes, all blocks are updated with new data if they have busy, read-only, or read-write permission.

Event declarations

Next, we need to declare all of the events that are triggered by incoming messages for this cache controller. These events come directly from the first row in Table 8.3 in Sorin et al.

enumeration(Event, desc="Cache events") {
    // From the processor/sequencer/mandatory queue
    Load,           desc="Load from processor";
    Store,          desc="Store from processor";

    // Internal event (only triggered from processor requests)
    Replacement,    desc="Triggered when block is chosen as victim";

    // Forwarded request from other cache via dir on the forward network
    FwdGetS,        desc="Directory sent us a request to satisfy GetS. We must have the block in M to respond to this.";
    FwdGetM,        desc="Directory sent us a request to satisfy GetM. We must have the block in M to respond to this.";
    Inv,            desc="Invalidate from the directory.";
    PutAck,         desc="Response from directory after we issue a put. This must be on the fwd network to avoid deadlock.";

    // Responses from directory
    DataDirNoAcks,  desc="Data from directory (acks = 0)";
    DataDirAcks,    desc="Data from directory (acks > 0)";

    // Responses from other caches
    DataOwner,      desc="Data from owner";
    InvAck,         desc="Invalidation ack from other cache after Inv";

    // Special event to simplify implementation
    LastInvAck,     desc="Triggered after the last ack is received";
}

User-defined structures

Next, we need to define some structures that we will use in other places in this controller. The first one we will define is Entry. This is the structure that is stored in the CacheMemory. It only needs to contain data and a state, but it may contain any other data you want. Note: The state that this structure is storing is the State type that was defined above, not a hardcoded state type.

You can find the abstract version of this class (AbstractCacheEntry) in src/mem/ruby/slicc_interface/AbstractCacheEntry.hh. If you want to use any of the member functions of AbstractCacheEntry, you need to declare them here (this isn’t used in this protocol).

structure(Entry, desc="Cache entry", interface="AbstractCacheEntry") {
    State CacheState,        desc="cache state";
    DataBlock DataBlk,       desc="Data in the block";
}

Another structure we will need is a TBE. TBE is the “transaction buffer entry”. This stores information needed during transient states. This is like an MSHR. It functions as an MSHR in this protocol, but the entry is also allocated for other uses. In this protocol, it will store the state (usually needed), data (also usually needed), and the number of acks that this block is currently waiting for. The AcksOutstanding is used for the transitions where other controllers send acks instead of the data.

structure(TBE, desc="Entry for transient requests") {
    State TBEState,         desc="State of block";
    DataBlock DataBlk,      desc="Data for the block. Needed for MI_A";
    int AcksOutstanding, default=0, desc="Number of acks left to receive.";
}

Next, we need a place to store all of the TBEs. This is an externally defined class; it is defined in C++ outside of SLICC. Therefore, we need to declare that we are going to use it, and also declare any of the functions that we will call on it. You can find the code for the TBETable in src/mem/ruby/structures/TBETable.hh. It is templatized on the TBE structure defined above, which gets a little confusing, as we will see.

structure(TBETable, external="yes") {
  TBE lookup(Addr);
  void allocate(Addr);
  void deallocate(Addr);
  bool isPresent(Addr);
}

The external="yes" tells SLICC to not look for the definition of this structure. This is similar to declaring a variable extern in C/C++.

Other declarations and definitions required

Finally, we are going to go through some boilerplate of declaring variables, declaring functions in AbstractController that we will use in this controller, and defining abstract functions in AbstractController.

First, we need to have a variable that stores a TBE table. We have to do this in SLICC because it is not until this time that we know the true type of the TBE table since the TBE type was defined above. This is some particularly tricky (or nasty) code to get SLICC to generate the right C++ code. The difficulty is that we want templatize TBETable based on the TBE type above. The key is that SLICC mangles the names of all types declared in the machine with the machine’s name. For instance, TBE is actually L1Cache_TBE in C++.

We also want to pass a parameter to the constructor of the TBETable. This is a parameter that is actually part of the AbstractController, thus we need to use the C++ name for the variable since it doesn’t have a SLICC name.

TBETable TBEs, template="<L1Cache_TBE>", constructor="m_number_of_TBEs";

If you can understand the above code, then you are an official SLICC ninja!

Next, any functions that are part of AbstractController need to be declared, if we are going to use them in the rest of the file. In this case, we are only going to use clockEdge():

Tick clockEdge();

There are a few other functions we’re going to use in actions. These functions are used in actions to set and unset implicit variables available in action code-blocks. Action code blocks will be explained in detail in the action section <MSI-actions-section>. These may be needed when a transition has many actions.

void set_cache_entry(AbstractCacheEntry a);
void unset_cache_entry();
void set_tbe(TBE b);
void unset_tbe();

Another useful function is mapAddressToMachine. This allows us to change the address mappings for banked directories or caches at runtime so we don’t have to hardcode them in the SLICC file.

MachineID mapAddressToMachine(Addr addr, MachineType mtype);

Finally, you can also add any functions you may want to use in the file and implement them here. For instance, it is convenient to access cache blocks by address with a single function. Again, in this function there is some SLICC trickery. We need to access “by pointer” since the cache block is something that we need to be mutable later (“by reference” would have been a better name). The cast is also necessary since we defined a specific Entry type in the file, but the CacheMemory holds the abstract type.

// Convenience function to look up the cache entry.
// Needs a pointer so it will be a reference and can be updated in actions
Entry getCacheEntry(Addr address), return_by_pointer="yes" {
    return static_cast(Entry, "pointer", cacheMemory.lookup(address));
}

The next set of boilerplate code rarely changes between different protocols. There’s a set of functions that are pure-virtual in AbstractController that we must implement.

getState
Given a TBE, cache entry, and address return the state of the block. This is called on the block to decide which transition to execute when an event is triggered. Usually, you return the state in the TBE or cache entry, whichever is valid.
setState
Given a TBE, cache entry, and address make sure the state is set correctly on the block. This is called at the end of the transition to set the final state on the block.
getAccessPermission
Get the access permission of a block. This is used during functional access to decide whether or not to functionally access the block. It is similar to getState, get the information from the TBE if valid, cache entry, if valid, or the block is not present.
setAccessPermission
Like getAccessPermission, but sets the permission.
functionalRead
Functionally read the data. It is possible the TBE has more up-to-date information, so check that first. Note: testAndRead/Write defined in src/mem/ruby/slicc_interface/Util.hh
functionalWrite
Functionally write the data. Similarly, you may need to update the data in both the TBE and the cache entry.
State getState(TBE tbe, Entry cache_entry, Addr addr) {
    // The TBE state will override the state in cache memory, if valid
    if (is_valid(tbe)) { return tbe.TBEState; }
    // Next, if the cache entry is valid, it holds the state
    else if (is_valid(cache_entry)) { return cache_entry.CacheState; }
    // If the block isn't present, then it's state must be I.
    else { return State:I; }
}

void setState(TBE tbe, Entry cache_entry, Addr addr, State state) {
  if (is_valid(tbe)) { tbe.TBEState := state; }
  if (is_valid(cache_entry)) { cache_entry.CacheState := state; }
}

AccessPermission getAccessPermission(Addr addr) {
    TBE tbe := TBEs[addr];
    if(is_valid(tbe)) {
        return L1Cache_State_to_permission(tbe.TBEState);
    }

    Entry cache_entry := getCacheEntry(addr);
    if(is_valid(cache_entry)) {
        return L1Cache_State_to_permission(cache_entry.CacheState);
    }

    return AccessPermission:NotPresent;
}

void setAccessPermission(Entry cache_entry, Addr addr, State state) {
    if (is_valid(cache_entry)) {
        cache_entry.changePermission(L1Cache_State_to_permission(state));
    }
}

void functionalRead(Addr addr, Packet *pkt) {
    TBE tbe := TBEs[addr];
    if(is_valid(tbe)) {
        testAndRead(addr, tbe.DataBlk, pkt);
    } else {
        testAndRead(addr, getCacheEntry(addr).DataBlk, pkt);
    }
}

int functionalWrite(Addr addr, Packet *pkt) {
    int num_functional_writes := 0;

    TBE tbe := TBEs[addr];
    if(is_valid(tbe)) {
        num_functional_writes := num_functional_writes +
            testAndWrite(addr, tbe.DataBlk, pkt);
        return num_functional_writes;
    }

    num_functional_writes := num_functional_writes +
            testAndWrite(addr, getCacheEntry(addr).DataBlk, pkt);
    return num_functional_writes;
}