Liedke’s Suggestions for Avoiding TLB Flush – Georgia Tech – Advanced Operating Systems

Liedtke, the author of the L3 microkernal, suggests tricks for exploiting the hardware and avoiding TLB flushes, even if the TLB is not address space tagged. And in particular, Liedtke’s suggestion is that take advantage of whatever the architecture offers you. For example, the architecture may offer segment registers in x86 and PowerPC, both of them offer segment registers. What the segment registers do is give an opportunity for the operating system to specify the range of virtual addresses that can be legally accessed by the currently running process. What that means is, even though you have a linear address space, that is, a linear virtual address space. You can carve out that linear virtual address space among several different protection domains by using segment registers. So, here is the linear address space provided by the hardware, starts from zero to a max and that max is, of course, decided by the number of bits you have for addressing in the hardware architecture. If it’s a 32 bit architecture, you have 2 to the 32 as the maximum addressing capability of that particular processor. If you have 64 bits, you have 2 to the 64 as a maximum address space that’s available in that particular architecture. So that’s the linear address space that is provided by the hardware. If the architecture, such as the PowerPC, offers segment registers to bound the range of virtual addresses that can be legally generated by a running process. Then use segment registers to define a protection domain. So here is one protection domain, S1, and it uses segment registers to say that this particular protection domain can generate virtual addresses starting from here to here. Any other virtual address generated by this guy is illegal, and the hardware will check that, because the segment registers are hardware-provided facility for bonding the range of legal virtual addresses that can be generated by this protection domain. Similarly, another protection domain, S2, can use the segment registers to carve out a different portion of the linear hardware address space. So, for S2, the bounds for legal, virtual addresses that can be generated by, a thread that is running, in this protect domain, starts from here. Ends here. And so on. So, in other words, we can take the hardware address space that’s available. And using segment registers, provided by the hardware, you can carve out the hardware address space into these regions. And once we do that, there is no need for flushing the TLB on a context switch. Because, even before we consult the TLB to see if there is a match for a particular virtual address in the TLB, segment register will act as a first line of defense. And say that, oh, this address is kosher. This address is not kosher because it is not within the bounds of legal addresses that can be generated by this protection domain. In other words, the segment bounds that we’re talking about are hardware enforced, so this works really well. If these protection domains are fairly small, meaning that the amount of space, memory space, that is needed by any given protection domain, is not the entire hardware address space. But it is a portion of the hardware address space. So we’re able to carve out the available hardware address space among multiple co-resident, protection domains, in the hardware address space, using these concept of segment registers.

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