= Invalidating TLB entries
When the process changes, `cr3` change to point to the page table of the new current process.
This creates a problem: the TLB is now filled with a bunch of cached entries for the old process.
A simple and naive solution would be to completely invalidate the TLB whenever the `cr3` changes.
However, this is would not be very efficient, because it often happens that we switch back to process 1 before process 2 has completely used up the entire TLB cache entries.
The solution for this is to use so called "Address Space Identifiers" (ASID) as mentioned in sources such as:
* https://stackoverflow.com/questions/52813239/how-many-bits-there-are-in-a-tlb-asid-tag-for-intel-processors-and-how-to-handl
* https://stackoverflow.com/questions/52713940/purpose-of-address-spaced-identifiersasids
* https://www.inf.ed.ac.uk/teaching/courses/os/slides/10-paging16.pdf
Basically, the OS assigns a different ASID for each process, and then TLB entries are automatically also tagged with that ASID. This way when the process makes an access, the TLB can determine if a hit is actually for the current process, or if it is an old address coincidence with another process.
The x86 also offers the `invlpg` instruction which explicitly invalidates a single TLB entry. Other architectures offer even more instructions to invalidated TLB entries, such as invalidating all entries on a given range.
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