"Process Identifier" Support in x86-64/x64 CPUs TLB

Recent x86-64 CPUs--at least from Intel--supports a sort of process identifier in the Translation Look-aside Buffer (TLB). This is important for multiprocessing in user mode, i.e. supporting more than one process running in user mode and switching between them without invalidating and flushing the TLB.

In ARMv6 architecture and above, there is an Application Space IDentifier (ASID) hardware that identifies the process which owns an entry in the TLB. This kind of support in x86/x86-64 CPUs is long overdue. Very probably because of the need to preserve compatibility with older OS running on the platform.

However, in newer x86-64 CPUs, there is a new "field" in the CR4 control register called Process-Context IDentifier Enable (PCIDE). This field controls whether Process-Context IDentifier (PCID) is enabled in the TLB or not. The value of the PCID for the "current" process is stored in CR3 register, in bits 0 to 11 (12-bits in total). This implies that the TLB can store up-to 4KB unique PCIDs.

You can read Intel64 Software Developer Manual Volume 3, section 4.10.1 Process-Context Identifiers (PCIDs) for the details. These two articles are also helpful:
http://linuxeco.com/?p=488
http://linuxeco.com/?page_id=2

PS: Thanks to Johan over at linkedin for the links ;-)

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