world switching, what does it mean?

[kaddkaka]

In the privileged documentation, the word "world" is mentioned five times, usually as "world switching". I don't see this described anywhere in the documentation. What is it? Should the document be amended in this regard?

[wmat]

On Tue, Jul 8, 2025 at 3:22 AM David Moberg ***@***.***> wrote: In the privileged documentation, the word "world" is mentioned five times, usually as "world switching". I don't see this described anywhere in the documentation. What is it? Should the document be amended in this regard?

A "world" is basically a privileged mode/level such as User mode, machine mode, and supervisor mode, so world switching means to change privilege levels thereby allowing or disallowing access to specific functionality. This sounds like a great candidate for a glossary entry

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[kaddkaka]

Hmm. Then why isn't it called "mode switching" or "switching privilege mode"?

[wmat]

On Tue, Jul 8, 2025 at 9:27 AM David Moberg ***@***.***> wrote: Hmm. Then why isn't it called "mode switching" or "switching privilege mode"?

I believe it's a common term within the security domain of ISA design. I've seen it used in the ARM specs before. If I'm incorrect, I'm happy to be corrected. —

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[gfavor]

World switches are switches not between privilege modes, but between what I (from my ARM past) would call translation regimes - where a translation regime is a set of privilege modes that share a common set of MMU (aka virtual memory) address translation tables. So, for example, S/HS and U modes are one translation regime, and a guest VM (with its VS and VU modes) is a translation regime.

So world switches occur when one switches from using one set of address translation tables to using a different set.

[aswaterman]

And in particular the term tends to be used to mean the act of switching which guest OS is running on a hypervisor (and hence which G-stage page tables are in effect); it is generally not used to describe changing which process is running on an OS (even though doing so requires changing which [V]S-stage page tables are in effect).

[gfavor]

Yes. I meant to mention that distinction but forgot to. In general, switching between privilege modes and switching between processes within a "translation regime", stays within that world. The key point being that even though part of the translation tables change when switching between processes, it is still the same overall set of translation tables being managed by the OS (in the case of S/HS+U modes). Whereas each VM has its own completely separate hgatp and associated translation tables.

[gfavor]

Closing since I believe the question has been sufficiantly answered/clarified.