xref: /gem5/src/mem/snoop_filter.cc

/*
 * Copyright (c) 2013-2015 ARM Limited
 * All rights reserved
 *
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 * to a hardware implementation of the functionality of the software
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 * met: redistributions of source code must retain the above copyright
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 * neither the name of the copyright holders nor the names of its
 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
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 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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 * Authors: Stephan Diestelhorst <stephan.diestelhorst@arm.com>
 */

/**
 * @file
 * Definition of a snoop filter.
 */

#include "base/misc.hh"
#include "base/trace.hh"
#include "debug/SnoopFilter.hh"
#include "mem/snoop_filter.hh"
#include "sim/system.hh"

std::pair<SnoopFilter::SnoopList, Cycles>
SnoopFilter::lookupRequest(const Packet* cpkt, const SlavePort& slave_port)
{
    DPRINTF(SnoopFilter, "%s: packet src %s addr 0x%x cmd %s\n",
            __func__, slave_port.name(), cpkt->getAddr(), cpkt->cmdString());

    // Ultimately we should check if the packet came from an
    // allocating source, not just if the port is snooping
    bool allocate = !cpkt->req->isUncacheable() && slave_port.isSnooping();
    Addr line_addr = cpkt->getBlockAddr(linesize);
    SnoopMask req_port = portToMask(slave_port);
    auto sf_it = cachedLocations.find(line_addr);
    bool is_hit = (sf_it != cachedLocations.end());

    // If the snoop filter has no entry, and we should not allocate,
    // do not create a new snoop filter entry, simply return a NULL
    // portlist.
    if (!is_hit && !allocate)
        return snoopDown(lookupLatency);

    // Create a new element through operator[] and modify in-place
    SnoopItem& sf_item = is_hit ? sf_it->second : cachedLocations[line_addr];
    SnoopMask interested = sf_item.holder | sf_item.requested;

    totRequests++;
    if (is_hit) {
        // Single bit set -> value is a power of two
        if (isPow2(interested))
            hitSingleRequests++;
        else
            hitMultiRequests++;
    }

    DPRINTF(SnoopFilter, "%s:   SF value %x.%x\n",
            __func__, sf_item.requested, sf_item.holder);

    if (allocate && cpkt->needsResponse()) {
        if (!cpkt->memInhibitAsserted()) {
            // Max one request per address per port
            panic_if(sf_item.requested & req_port, "double request :( "\
                     "SF value %x.%x\n", sf_item.requested, sf_item.holder);

            // Mark in-flight requests to distinguish later on
            sf_item.requested |= req_port;
        } else {
            // NOTE: The memInhibit might have been asserted by a cache closer
            // to the CPU, already -> the response will not be seen by this
            // filter -> we do not need to keep the in-flight request, but make
            // sure that we know that that cluster has a copy
            panic_if(!(sf_item.holder & req_port), "Need to hold the value!");
            DPRINTF(SnoopFilter, "%s:   not marking request. SF value %x.%x\n",
                    __func__,  sf_item.requested, sf_item.holder);
        }
        DPRINTF(SnoopFilter, "%s:   new SF value %x.%x\n",
                __func__,  sf_item.requested, sf_item.holder);
    }
    return snoopSelected(maskToPortList(interested & ~req_port), lookupLatency);
}

void
SnoopFilter::updateRequest(const Packet* cpkt, const SlavePort& slave_port,
                           bool will_retry)
{
    DPRINTF(SnoopFilter, "%s: packet src %s addr 0x%x cmd %s\n",
            __func__, slave_port.name(), cpkt->getAddr(), cpkt->cmdString());

    // Ultimately we should check if the packet came from an
    // allocating source, not just if the port is snooping
    bool allocate = !cpkt->req->isUncacheable() && slave_port.isSnooping();
    if (!allocate)
        return;

    Addr line_addr = cpkt->getBlockAddr(linesize);
    SnoopMask req_port = portToMask(slave_port);
    SnoopItem& sf_item  = cachedLocations[line_addr];

    DPRINTF(SnoopFilter, "%s:   old SF value %x.%x retry: %i\n",
            __func__, sf_item.requested, sf_item.holder, will_retry);

    if (will_retry) {
        // Unmark a request that will come again.
        sf_item.requested &= ~req_port;
        return;
    }

    // will_retry == false
    if (!cpkt->needsResponse()) {
        // Packets that will not evoke a response but still need updates of the
        // snoop filter; WRITEBACKs for now only
        if (cpkt->cmd == MemCmd::Writeback) {
            // make sure that the sender actually had the line
            panic_if(sf_item.requested & req_port, "double request :( "\
                     "SF value %x.%x\n", sf_item.requested, sf_item.holder);
            panic_if(!(sf_item.holder & req_port), "requester %x is not a "\
                     "holder :( SF value %x.%x\n", req_port,
                     sf_item.requested, sf_item.holder);
            // Writebacks -> the sender does not have the line anymore
            sf_item.holder &= ~req_port;
        } else {
            // @todo Add CleanEvicts
            assert(cpkt->cmd == MemCmd::CleanEvict);
        }
        DPRINTF(SnoopFilter, "%s:   new SF value %x.%x\n",
                __func__,  sf_item.requested, sf_item.holder);
    }
}

std::pair<SnoopFilter::SnoopList, Cycles>
SnoopFilter::lookupSnoop(const Packet* cpkt)
{
    DPRINTF(SnoopFilter, "%s: packet addr 0x%x cmd %s\n",
            __func__, cpkt->getAddr(), cpkt->cmdString());

    assert(cpkt->isRequest());

    // Broadcast / filter upward snoops
    const bool filter_upward = true;  // @todo: Make configurable

    if (!filter_upward)
        return snoopAll(lookupLatency);

    Addr line_addr = cpkt->getBlockAddr(linesize);
    auto sf_it = cachedLocations.find(line_addr);
    bool is_hit = (sf_it != cachedLocations.end());
    // Create a new element through operator[] and modify in-place
    SnoopItem& sf_item = is_hit ? sf_it->second : cachedLocations[line_addr];

    DPRINTF(SnoopFilter, "%s:   old SF value %x.%x\n",
            __func__, sf_item.requested, sf_item.holder);

    SnoopMask interested = (sf_item.holder | sf_item.requested);

    totSnoops++;
    if (is_hit) {
        // Single bit set -> value is a power of two
        if (isPow2(interested))
            hitSingleSnoops++;
        else
            hitMultiSnoops++;
    }
    // ReadEx and Writes require both invalidation and exlusivity, while reads
    // require neither. Writebacks on the other hand require exclusivity but
    // not the invalidation. Previously Writebacks did not generate upward
    // snoops so this was never an aissue. Now that Writebacks generate snoops
    // we need to special case for Writebacks.
    assert(cpkt->cmd == MemCmd::Writeback ||
           (cpkt->isInvalidate() == cpkt->needsExclusive()));
    if (cpkt->isInvalidate() && !sf_item.requested) {
        // Early clear of the holder, if no other request is currently going on
        // @todo: This should possibly be updated even though we do not filter
        // upward snoops
        sf_item.holder = 0;
    }

    DPRINTF(SnoopFilter, "%s:   new SF value %x.%x interest: %x \n",
            __func__, sf_item.requested, sf_item.holder, interested);

    return snoopSelected(maskToPortList(interested), lookupLatency);
}

void
SnoopFilter::updateSnoopResponse(const Packet* cpkt,
                                 const SlavePort& rsp_port,
                                 const SlavePort& req_port)
{
    DPRINTF(SnoopFilter, "%s: packet rsp %s req %s addr 0x%x cmd %s\n",
            __func__, rsp_port.name(), req_port.name(), cpkt->getAddr(),
            cpkt->cmdString());

    assert(cpkt->isResponse());
    assert(cpkt->memInhibitAsserted());

    // Ultimately we should check if the packet came from an
    // allocating source, not just if the port is snooping
    bool allocate = !cpkt->req->isUncacheable() && req_port.isSnooping();
    if (!allocate)
        return;

    Addr line_addr = cpkt->getBlockAddr(linesize);
    SnoopMask rsp_mask = portToMask(rsp_port);
    SnoopMask req_mask = portToMask(req_port);
    SnoopItem& sf_item = cachedLocations[line_addr];

    DPRINTF(SnoopFilter, "%s:   old SF value %x.%x\n",
            __func__,  sf_item.requested, sf_item.holder);

    // The source should have the line
    panic_if(!(sf_item.holder & rsp_mask), "SF value %x.%x does not have "\
             "the line\n", sf_item.requested, sf_item.holder);

    // The destination should have had a request in
    panic_if(!(sf_item.requested & req_mask), "SF value %x.%x missing "\
             "the original request\n",  sf_item.requested, sf_item.holder);

    // Update the residency of the cache line.
    if (cpkt->needsExclusive() || !cpkt->sharedAsserted()) {
        DPRINTF(SnoopFilter, "%s:  dropping %x because needs: %i shared: %i "\
                "SF val: %x.%x\n", __func__,  rsp_mask,
                cpkt->needsExclusive(), cpkt->sharedAsserted(),
                sf_item.requested, sf_item.holder);

        sf_item.holder &= ~rsp_mask;
        // The snoop filter does not see any ACKs from non-responding sharers
        // that have been invalidated :(  So below assert would be nice, but..
        //assert(sf_item.holder == 0);
        sf_item.holder = 0;
    }
    assert(cpkt->cmd != MemCmd::Writeback);
    sf_item.holder |=  req_mask;
    sf_item.requested &= ~req_mask;
    DPRINTF(SnoopFilter, "%s:   new SF value %x.%x\n",
            __func__, sf_item.requested, sf_item.holder);
}

void
SnoopFilter::updateSnoopForward(const Packet* cpkt,
        const SlavePort& rsp_port, const MasterPort& req_port)
{
    DPRINTF(SnoopFilter, "%s: packet rsp %s req %s addr 0x%x cmd %s\n",
            __func__, rsp_port.name(), req_port.name(), cpkt->getAddr(),
            cpkt->cmdString());

    Addr line_addr = cpkt->getBlockAddr(linesize);
    SnoopItem& sf_item = cachedLocations[line_addr];
    SnoopMask rsp_mask M5_VAR_USED = portToMask(rsp_port);

    assert(cpkt->isResponse());
    assert(cpkt->memInhibitAsserted());

    DPRINTF(SnoopFilter, "%s:   old SF value %x.%x\n",
            __func__,  sf_item.requested, sf_item.holder);

    // Remote (to this snoop filter) snoops update the filter already when they
    // arrive from below, because we may not see any response.
    if (cpkt->needsExclusive()) {
        // If the request to this snoop response hit an in-flight transaction,
        // the holder was not reset -> no assertion & do that here, now!
        //assert(sf_item.holder == 0);
        sf_item.holder = 0;
    }
    DPRINTF(SnoopFilter, "%s:   new SF value %x.%x\n",
            __func__, sf_item.requested, sf_item.holder);
}

void
SnoopFilter::updateResponse(const Packet* cpkt, const SlavePort& slave_port)
{
    DPRINTF(SnoopFilter, "%s: packet src %s addr 0x%x cmd %s\n",
            __func__, slave_port.name(), cpkt->getAddr(), cpkt->cmdString());

    assert(cpkt->isResponse());

    // Ultimately we should check if the packet came from an
    // allocating source, not just if the port is snooping
    bool allocate = !cpkt->req->isUncacheable() && slave_port.isSnooping();
    if (!allocate)
        return;

    Addr line_addr = cpkt->getBlockAddr(linesize);
    SnoopMask slave_mask = portToMask(slave_port);
    SnoopItem& sf_item = cachedLocations[line_addr];

    DPRINTF(SnoopFilter, "%s:   old SF value %x.%x\n",
            __func__,  sf_item.requested, sf_item.holder);

    // Make sure we have seen the actual request, too
    panic_if(!(sf_item.requested & slave_mask), "SF value %x.%x missing "\
             "request bit\n", sf_item.requested, sf_item.holder);

    // Update the residency of the cache line.
    if (cpkt->needsExclusive() || !cpkt->sharedAsserted())
        sf_item.holder = 0;
    sf_item.holder |=  slave_mask;
    sf_item.requested &= ~slave_mask;
    DPRINTF(SnoopFilter, "%s:   new SF value %x.%x\n",
            __func__, sf_item.requested, sf_item.holder);
}

void
SnoopFilter::regStats()
{
    totRequests
        .name(name() + ".tot_requests")
        .desc("Total number of requests made to the snoop filter.");

    hitSingleRequests
        .name(name() + ".hit_single_requests")
        .desc("Number of requests hitting in the snoop filter with a single "\
              "holder of the requested data.");

    hitMultiRequests
        .name(name() + ".hit_multi_requests")
        .desc("Number of requests hitting in the snoop filter with multiple "\
              "(>1) holders of the requested data.");

    totSnoops
        .name(name() + ".tot_snoops")
        .desc("Total number of snoops made to the snoop filter.");

    hitSingleSnoops
        .name(name() + ".hit_single_snoops")
        .desc("Number of snoops hitting in the snoop filter with a single "\
              "holder of the requested data.");

    hitMultiSnoops
        .name(name() + ".hit_multi_snoops")
        .desc("Number of snoops hitting in the snoop filter with multiple "\
              "(>1) holders of the requested data.");
}

SnoopFilter *
SnoopFilterParams::create()
{
    return new SnoopFilter(this);
}