tdx_mcall_extend_rtmr(9)

💖 tdx_mcall_extend_rtmr — a Kernel API Manual moment (section 9, bestie) 💖

okay so real talk, this is a page from the Kernel API Manual, and it's all about tdx_mcall_extend_rtmr — which is literally just a wrapper to extend RTMR registers using the TDG.MR.RTMR.EXTEND TDCALL. that's the whole vibe. let's get into it ✨

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🌸 SYNOPSIS 🌸

int tdx_mcall_extend_rtmr (u8 index , u8 *data );

like, that's the signature. memorize it. it's giving "small function, big responsibilities" 🫶

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🎀 ARGUMENTS 🎀

• index — the index of the RTMR register you wanna extend. yes, you have to tell it which one. no vibes-based guessing allowed 💅
• data — the address of the input buffer holding the RTMR register extend data. it's a u8 *, so it's bytes, and it's your job to make sure they're the right bytes bestie 🙈

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🦋 DESCRIPTION 🦋

so for the actual lore: go read the section titled "TDG.MR.RTMR.EXTEND leaf" in the TDX Module v1.0 specification — that's where the real info on the TDG.MR.RTMR.EXTEND TDCALL lives. this page is just the wrapper energy, the spec is the main character 📖✨

it's used in the TDX guest driver module to let the user extend the RTMR registers. that's the whole purpose. iconic, frankly 👑

and here's the return value tea ☕ — pay attention because this is the part people mess up:

• returns 0 on success (slay 💚)
• -ENXIO for invalid operands (you gave it nonsense, bestie 😭)
• -EBUSY for busy operation (it's booked and busy, try again later 💀)
• -EIO on other TDCALL failures (the generic "something went sideways" of it all)

ngl, keep an eye on those negative returns — a 0 is the only one that means you actually ate 🔥

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⭐ SEE ALSO ⭐

go peek at these, they're all part of the same friend group:

• the kernel file ./arch/x86/coco/tdx/tdx.c
• tdx_mcall_get_report0(9)
• tdx_hcall_get_quote(9)

iykyk 💜

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and that's the page!! huge shoutout to the folks over at ./arch/x86/coco/tdx for shipping this in October 2026 — periodt, go extend some RTMR registers responsibly 🧚‍♀️💖🎀