How TLB misses are handled (part 2)

18 May, 2010 § 1 Comment

This blog post was co-authored by Brendan Grebur and Jared Wein.

As a follow up from the previous post on TLB misses, I’d like to cover a special case of TLB misses.

At boot time there is a “chicken-and-egg” dilemma where the TLB is empty yet TLB values are needed immediately.

How does a computer handle this? First to make some assumptions:

  • Linux Kernel
  • Hardware-managed TLB

The x86 chips boot up in Real Mode with a very limited memory space and the MMU disabled. The Linux kernel is uncompressed and loaded into low-memory by the boot loader. Assembly code initializes a page directory for the initial kernel process, sets the CR3 register, then enables the PG bit in CR0 to effectively enable the MMU and begin addressing in Protected Mode. Since this area is kernel memory, the virtual address will be identical to the physical address, as kernel memory is never swapped out. The init process begins running C code and making memory references to initialize the rest of the kernel. TLB misses occur, but resolve themselves as the MMU walks the page directory previously set up for the kernel process.

Sources:

  1. http://lkml.indiana.edu/hypermail/linux/kernel/0811.2/01602.html
  2. http://beaversource.oregonstate.edu/projects/cspfl/browser/trunk/software/u-boot-omap3/board/sbc8560/tlb.c?rev=376
  3. http://tldp.org/LDP/tlk/tlk.html
  4. http://lxr.linux.no/linux-old+v2.0.33/arch/i386/kernel/head.S#L286

Tagged: , ,

§ One Response to How TLB misses are handled (part 2)

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

What’s this?

You are currently reading How TLB misses are handled (part 2) at JAWS.

meta

Follow

Get every new post delivered to your Inbox.

Join 982 other followers