set_mtrr(9)

💅 set_mtrr(9) — like, the Kernel API Manual entry, bestie ✨

okay so this is set_mtrr(9), and honestly? it's giving "trust me, the hardware vendor wrote the instructions for us" energy. let's get into it 🎀

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🌸 NAME 🌸

set_mtrr — update mtrrs on all the processors. every single one. no cap 💖

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✨ SYNOPSIS ✨

void set_mtrr (unsigned int reg , unsigned long base , unsigned long
size , mtrr_type type );

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🦋 ARGUMENTS 🦋

• reg — which mtrr we're talking about, the one in question 💅
• base — the mtrr base, periodt
• size — the mtrr size, as one does
• type — the mtrr type 🌈

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🔥 DESCRIPTION 🔥

ngl, this one is kinda tricky 😭 but fortunately Intel spelled it out for us cleanly, so we don't have to vibe-check it ourselves:

1. Queue work to do the following on all processors:
2. Disable Interrupts
3. Wait for all procs to do so
4. Enter no-fill cache mode
5. Flush caches
6. Clear PGE bit
7. Flush all TLBs
8. Disable all range registers
9. Update the MTRRs
10. Enable all range registers
11. Flush all TLBs and caches again
12. Enter normal cache mode and reenable caching
13. Set PGE
14. Wait for buddies to catch up
15. Enable interrupts.

like… that's the whole choreo 💀 and yes it's a numbered list inside the description, we don't make the rules, Intel does.

so what does that actually mean for us? well, stop_machine will ensure that the rendezvous handler is started on each CPU 👑 and in lockstep they do the state transition of disabling interrupts, updating MTRR's (the CPU vendors may each do it differently, so we call mtrr_if->set callback and let them take care of it.) and enabling interrupts. iconique, honestly ✨

💜 note that the mechanism is the same for UP systems, too; all the SMP stuff becomes nops. so it's literally the same dance, the moves just turn into nothing. ate and left no crumbs 🫶

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🧚 SEE ALSO 🧚

• Kernel file ./arch/x86/kernel/cpu/mtrr/mtrr.c
• mtrr_rendezvous_handler(9)
• mtrr_add_page(9)
• mtrr_add(9)
• mtrr_del_page(9)
• mtrr_del(9)
• arch_phys_wc_add(9)
• mtrr_bp_init(9)
• mtrr_save_state(9)

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k bye bestie 🫶 this page hails from ./arch/x86/kernel/cpu/mtrr, dated October 2026, and it's set_mtrr(9) signing off — mwah 💋