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School of Computer Science & Engineering
University of New South Wales
 Advanced Operating Systems 
 COMP9242 2002/S2 

Virtual Cache Issues

Homonyms:

• Same VA corresponds to several PAs
  • standard situation in multitasking systems (not SASOS!)
• Problem: tag may not uniquely identify cache data!
• Homonyms lead to cache accessing the wrong data!
• Homonym prevention:
  • tag virtual address with address-space ID (ASID)
    • disambiguates virtual addresses (makes them globally valid)
  • use physical tags
  • flush cache on context switch
  • use a SASOS

Synonyms (aliases):

• Several VAs map to the same PA
  • frames shared between processes
  • multiple mappings of a frame within an address space
• Synonyms in cache may lead to accessing stale data:
  • same data may be cached in several lines
  • on write, one synonym is update
  • a subsequent read on the other synonym returns the old value
• Physical tagging doesn't help!
• ASIDs don't help either!
• Whether synonyms are a problem depends on cache size and page size!

Example: MIPS R4x00 synonyms

• Virtually-indexed, physically tagged on-chip L1 cache
  • 16kB cache with 32B lines
  • 4kB (base) page size

• Remember, location of data in cache determined by index
• Tag only confirms whether it's a hit!
• Synonym problem iff VA VA
  

Synonyms...

• Avoiding synonym problems:
  • hardware synonym detection
  • flush cache on context switch
    • doesn't help for aliasing within address space
  • detect synonyms and ensure
    • all read-only, OR
    • only one synonym mapped
  • restrict VM mapping so synonyms map to same cache set
    • e.g., R4x00: ensure that VA PA