[gem5-users] Alpha-TLB Questions

[刘天义]

Hi all,


     I have 3 questions


Q1: Does the "lookup" function search the vpn in both TLB and DRAM?


     file: src/arch/alpha/tlb.cc
     function： 
             Fault  TLB::translateData(RequestPtr req, ThreadContext *tc, bool 
write)
    code： 
            // not a physical address: need to look up pte        
           TlbEntry *entry = lookup(VAddr(req->getVaddr()).vpn(), asn); 


Q2: What is EntryCache? What is lookuptable?  where does the  lookuptable 
reside, in TLB or DRAM?


     // look up an entry in the TLB
TlbEntry *
TLB::lookup(Addr vpn, uint8_t asn)
{
    // assume not found...
    TlbEntry *retval = NULL;


    if (EntryCache[0]) {
        if (vpn == EntryCache[0]->tag &&
            (EntryCache[0]->asma || EntryCache[0]->asn == asn))
            retval = EntryCache[0];
        else if (EntryCache[1]) {
            if (vpn == EntryCache[1]->tag &&
                (EntryCache[1]->asma || EntryCache[1]->asn == asn))
                retval = EntryCache[1];
            else if (EntryCache[2] && vpn == EntryCache[2]->tag &&
                     (EntryCache[2]->asma || EntryCache[2]->asn == asn))
                retval = EntryCache[2];
        }
    }


    if (retval == NULL) {
        PageTable::const_iterator i = lookupTable.find(vpn);
        if (i != lookupTable.end()) {
            while (i->first == vpn) {
                int index = i->second;
                TlbEntry *entry = &table[index];
                assert(entry->valid);
                if (vpn == entry->tag && (entry->asma || entry->asn == asn)) {
                    retval = updateCache(entry);
                    break;
                }
                ++i;
            }
        }
    }


Q3: if the entry is NULL, will this transition be deleted? If so, how this 
memory request will be dealt with in gem5?


if (!entry) {
                // page fault
                if (write) { write_misses++; } else { read_misses++; }
                uint64_t flags = (write ? MM_STAT_WR_MASK : 0) |
                    MM_STAT_DTB_MISS_MASK;
                return (req->getFlags() & AlphaRequestFlags::VPTE) ?
                    (Fault)(std::make_shared<PDtbMissFault>(req->getVaddr(),
                                                            req->getFlags(),
                                                            flags)) :
                    (Fault)(std::make_shared<NDtbMissFault>(req->getVaddr(),
                                                            req->getFlags(),
                                                            flags));
            }


Thank you very much.
Yi


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