__copy_xstate_to_uabi_buf(9)

💅 __copy_xstate_to_uabi_buf(9) — from the Kernel API Manual, bestie 🎀

okay so picture this: we're deep in the Kernel API Manual and this one function is literally the whole vibe. it's giving "translator between two worlds" and honestly? iconic. ✨

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🌸 NAME 🌸

__copy_xstate_to_uabi_buf — copy kernel saved xstate to a UABI buffer 🫶

like yes, that's exactly what it says on the tin, no cap.

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✨ SYNOPSIS ✨

void __copy_xstate_to_uabi_buf (struct membuf to , struct fpstate
*fpstate , u64 xfeatures , u32 pkru_val , enum xstate_copy_mode
copy_mode );

she takes five things and returns nothing (void, my beloved 💀). periodt.

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🦋 ARGUMENTS 🦋

• to — the membuf descriptor 💌 like, this is where the goods are going, bestie.
• fpstate — the fpstate buffer from which to copy. the source. the origin story. 🌱
• xfeatures — the mask of xfeatures to save. heads up: this one's XSAVE mode only, iykyk. 🎭
• pkru_val — the PKRU value to store in the PKRU component. she's specific like that. 🔥
• copy_mode — the requested copy mode 👑 whatever mode you asked for, that's the one you get.

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💖 DESCRIPTION 💖

okay so the whole gig here is conversion, and it's lowkey a glow-up. she converts from kernel XSAVE or XSAVES compacted format into the UABI conforming format 🧚 — meaning from the kernel's internal, hardware dependent storage format into the requested mode. so yeah, it's a format transformation and she ate. 💅

and this part is crucial, do not skip it: UABI XSTATE is always uncompacted! always. no exceptions. like the opposite of compacted, bestie. 🌈

she does support partial copy 🩷 but — and this is the plot twist — to.pos always starts from zero. always. every single time. 🙈

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⭐ SEE ALSO ⭐

go look at the kernel file ./arch/x86/kernel/fpu/xstate.c — that's the main character's home address 🏠💜

and then the whole friend group, fr fr:

• check_xtile_data_against_struct(9)
• copy_xstate_to_uabi_buf(9)
• copy_uabi_to_xstate(9)
• xsaves(9)
• xrstors(9)
• fpstate_realloc(9)
• fpu_xstate_prctl(9)

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ngl this one's short but she is doing the work. partial copy? supported. to.pos starting from zero? non-negotiable. UABI uncompacted? forever and always. 🍓

and that's the post! this lil page lives in ./arch/x86/kernel/fpu, dated October 2026 🌸 — mwah, thanks for reading, ily, bye bestie! 💕🎀🦋