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Re: Silly question - a further one : TLB flush

Vincent wrote:
According to a recent article published on, Unises' i386 kernels, although they can address the integrality of the 4 GiB memory space, both in user- and kernel mode, suffer from a TLB cache penalty each time a system call (trap) or interrupt is entered in, because the OS has to switch the MMU between user- and kernel VM space, and thus flush the TLB before reloading it. This penalty is not incured by 64-bits kernels, because the whole VM space can safely be divided between user and kernel space without any overlap, and thus the TLB can hold "global" VM page translation infos (what 32bit-Windows versions actually do, limiting user memory space to 2 or 3 GiB at most).
Could you provide me with a link to this article please?

What follows should be reviewed by gurus; but that's what I understood while browsing through netbsd's code. It may contain mistakes or misinterpretations.

For i386, netbsd uses a 3GB/1GB memory split and a flat address space (the descriptor table is loaded with a segment starting at address 0 and ending at the 4GB boundary). As a consequence, the MMU contains both user and kernel mappings, which do not require a TLB flush when switching contextes.

If you want to see what the memory layout is for a specific port, you should read the comments in pmap. They explain it all. IIRC, for windows, they use a 2GB/2GB memory split.

What you are describing happens for amd64 OSs running under Xen though. x86_64 removed the concept of segmentation, and left two rings, a privileged and unprivileged one, while i386 provides 4 ring levels. Since the hypervisor typically runs in privileged ring (aka ring 0), the guest OS is put in the unprivileged one (ring 3), both user and kernel space. So, you may have to update protections between user and kernel, which requires local TLB flushes.

Note that increasing the size and numbers of registers does have penalties, as you have to store them somewhere (stack) when context switching. This penalizes microkernels based OS, since you are frequently switching, compared to bigger, monolith ones.

This must be benchmarked though, as it is pure speculation from me. I have never run such a performance assessment myself.

Jean-Yves Migeon

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