fpu_sync_gu...it_xfd_state(9)

💅 fpu_sync_guest_vmexit_xfd_state(9)

okay bestie, so this is a page out of the Kernel API Manual, and it's about one very specific little function in ./arch/x86/kernel/fpu that keeps your XFD state from lying to you. yes, XFD. we're doing this. 🦋

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✨ NAME ✨

fpu_sync_guest_vmexit_xfd_state — bestie, it synchronizes the XFD MSR and the software state. that's the whole vibe. no cap. 💫

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🌸 SYNOPSIS 🌸

void fpu_sync_guest_vmexit_xfd_state (void );

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🎀 ARGUMENTS 🎀

• void — literally no arguments. nothing. zero. she takes nothing and she gives everything. 🫶

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🔥 DESCRIPTION 🔥

okay so here's the situation, and you need to hear this one. 💀

You must invoke this from KVM after a VMEXIT and before enabling interrupts, but only when XFD write emulation is disabled. like, that's the condition. that's the assignment. 🙈

and why? because the guest can freely modify XFD — she does whatever she wants, main character energy, no permission needed — so the state at VMEXIT is not guaranteed to be the same as the state on VMENTER. not even close. 😭 so before you go doing anything that depends on that software state, you gotta update the software state first. order matters, bestie. it's giving prerequisites. ✨

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💀 NOTE 💀

it can be invoked unconditionally even when write emulation is enabled — the price is just a then pointless MSR read. lowkey a scam, highkey harmless. 💸

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🌈 SEE ALSO 🌈

• Kernel file ./arch/x86/kernel/fpu/core.c — go read it, iykyk 📖
• fpu_enable_guest_xfd_features(9) — her sister function, equally iconic 👑

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that's the whole page, babe. go sync that state. slay. 💅

source: ./arch/x86/kernel/fpu · version/stamp: Kernel API Manual, October 2026 🎀💖