__x86_set_memory_region(9)

🌸 __x86_set_memory_region (9) — from the Kernel API Manual, bestie 💅

okay so like. this page is literally your cheat sheet for __x86_set_memory_region, a kernel function that sets up a KVM internal memory slot, and I'm gonna walk you through it like we're texting at 2am 🫶

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✨ NAME ✨

__x86_set_memory_region — Setup KVM internal memory slot 👑

like the whole personality of this function is "make a memory slot happen" and honestly? iconic.

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🦋 SYNOPSIS 🦋

void __user * __x86_set_memory_region (struct kvm *kvm , int id , gpa_t
gpa , u32 size );

that's the whole signature, no cap — returns a void __user *, takes four things. slay ✨

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🎀 ARGUMENTS 🎀

• kvm — the kvm pointer to the VM. your main character 🫶
• id — the slot ID to setup. you gotta know which slot you're messing with 💅
• gpa — the GPA to install the slot (unused when size == 0). like, if you're uninstalling, this one's just sitting there vibing 🙈
• size — the size of the slot. Set to zero to uninstall a slot. zero means bye-bye 👋

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💖 DESCRIPTION 💖

So this function helps to setup a KVM internal memory slot — that's literally its job, and it ate. 🍓

The rule is so simple it's giving elementary school: specify size > 0 to install a new slot, while size == 0 to uninstall a slot. that's it, that's the vibe check ✅

And the return code can be one of the following 👇

• HVA — on success (uninstall will return a bogus HVA). yes, on the uninstall path you get a bogus HVA, it's fake, it's not real, do not trust it 😭
• -errno — on error. the classic, the legend 💀

🚨 the caller should ALWAYS use IS_ERR to check the return value before use. no like seriously, always. don't just start using the thing, run it through IS_ERR first, that's the whole point, periodt. 💅

okay now the important lore 🧚: the KVM internal memory slots are guaranteed to remain valid and unchanged until the VM is destroyed — i.e., the GPA->HVA translation will not change. so that mapping? locked in. stable. not going anywhere until the VM goes poof. 🔥

BUT — and this is the plot twist — the HVA is a user address, i.e. its accessibility is not guaranteed, and must be accessed via __copy_{to,from}_user. so don't go dereferencing it directly like a fool, use the user-copy helpers, they exist for a reason 🌈

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⭐ SEE ALSO ⭐

• Kernel file ./arch/x86/kvm/x86.c — go read the source, iykyk 💜

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okay that's the whole page, you're so welcome 😌 this little beauty comes to you from ./arch/x86/kvm, dated October 2026 — so when you're deep in x86.c at some ungodly hour, just remember: IS_ERR first, __copy_{to,from}_user always, and you'll be fine. mwah 💖🎀👑