mtrr_add(9)

💅 NAME 💅

mtrr_add — add a memory type region ✨ (yes bestie, we're registering cache regions today, it's giving hardware whisperer)

This one lives in the Kernel API Manual, section 9 of the kernel docs — driver-dev territory, iykyk 🫶

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✨ SYNOPSIS ✨

int mtrr_add (unsigned long base , unsigned long size , unsigned int
       type , bool increment );

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🎀 ARGUMENTS 🎀

• base — the physical base address of the region, like the street address of your little memory neighborhood 🏡
• size — the physical size of the region, how many blocks we're claiming 📏
• type — which flavour of MTRR you want, the vibe of caching you're requesting 🍓
• increment — if this is true, do usage counting on the region (we keep tally, no cap 🧮)

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🦋 DESCRIPTION 🦋

okay so. Memory type region registers are the girlypops that control caching on newer Intel and non-Intel processors 💅 This function is literally how a driver slides into the kernel and requests that an MTRR gets added.

The details and hardware specifics of each processor's implementation? Completely hidden from the caller 🙈 The caller doesn't need to know. But — and this is the part you cannot skip — the caller should expect to need to provide a power of two size on an equivalent power of two boundary. That's the deal, bestie. Non-negotiable 💀

If the region can't be added, whether it's because all the regions are already in use or because the CPU just cannot support it, you get a negative value back 😭 no exceptions.

On success you get the register number for this entry returned — but lowkey treat that as a cookie only 🍪 Don't go reading meaning into it, it's just a little guy to hand back later.

On a multiprocessor machine, the changes are made to all processors. And on x86 this is straight up required by the Intel processors — everybody gets the update, periodt 👑

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🍬 the available types 🍬

• MTRR_TYPE_UNCACHABLE — no caching. None, nada, zero, the cold shoulder 🧊
• MTRR_TYPE_WRBACK — write data back in bursts whenever
• MTRR_TYPE_WRCOMB — write data back soon but allow bursts
• MTRR_TYPE_WRTHROUGH — cache reads but not writes (reads get the love, writes get nothing 🙈)

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💀 BUGS 💀

Needs a quiet flag for the cases where drivers do not mind failures and do not wish system log messages to be sent.

Like, sometimes a driver is fully fine with a failure and does not want the system log getting all loud and messy about it 🫠 We need a quiet flag for that. It's not here yet. Note it, love it, ship it.

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🌈 SEE ALSO 🌈

Kernel file ./arch/x86/kernel/cpu/mtrr/mtrr.c

mtrr_rendezvous_handler(9), set_mtrr(9), mtrr_add_page(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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💖 mwah, that's the whole page bestie 💖 this one comes to you courtesy of ./arch/x86/kernel/cpu/mtrr and the October 2026 edition of the kernel API manual — dated and documented with love, go add those regions responsibly 🎀 ✨ 🌸