Avoiding TLB Flushes on Context Switches on x86 Processors: The PCID

CR3, CR4 are Control registers on x86 processors that are used configure and manage protected-mode functionality. These instructions may only be executed at CPL(Current Privilege) == 0 . i.e. in Kernel Mode.
In the x86, linear addresses are translated by the TLB into Physical addresses (assuming Page Table Walks have been done prior to look up etc).
A Cr3 load switches out Page Tables. In the Linux Kernel, it is executed during scheduling new process at context switch time (context_switch). Before the days of the PCID (see below), a load of CR3 flushed the TLB.
Avoiding TLB flushes on Loads of CR3 are key to avoiding performace hits  on context switches.  In other words, a processor really needs to facilitate the storage of multiple address spaces in the TLB across context switches.
Process-context identifiers (PCIDs) are a facility in x86 processors  by which a logical processor may cache information for multiple linear-address spaces in the TLB.

The processor may retain cached information when software switches to a different linear-address space with a different PCID e.g., by loading CR3.

When using entries in the TLBs and paging-structure caches to translate a linear address, a logical processor uses only those entries associated with the current PCID

i.e. A PCID is a 12-bit identifier, and be thought of as a “Process-ID” for TLBs.

If CR4.PCIDE = 0 (but 17 of CR4), the current PCID is always 000H; otherwise, the current PCID is the value of bits 11:0 of CR3.

Non-zero PCIDs are enabled by setting the PCIDE flag (bit 17 of CR4). Rules do apply on enabling PCID on x86 processors. Caveats emptor. Naturally, restrictions on the operating system may apply to take advantage of this mechanism. Context switches that require isolation of Linear addresses between processes must be done  with care. And/or  linear addresses between processes with PCID‘s enabled may overlap, and so will translations of Linear Addresses to Physical memory ! Ouch.

More on the implications of this for Linux will follow.

I  explain Linux Kernel concepts and more in my classes ( Advanced Linux Kernel Programming @UCSC-Extension, and also in other classes that I teach independently). As always, Feedback, Questions  and Comments are appreciated and will be responded to.

About Anand

Anand is a veteran of Silicon Valley with development experience and patents that span Processors, Operating systems, Networking and Systems development. Anand has been working for the past few years with Service Providers and large Enterprises developing e and Training systems.

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