__cmpxchg_with_key(9)

✨ __cmpxchg_with_key ✨

okay bestie, so this one is a Kernel API Manual situation — section 9 of the kernel docs, no cap — and it's the page for __cmpxchg_with_key. she's a little niche, she's a little s390-specific, but she ate and we're gonna get into it 🫶

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💅 NAME

__cmpxchg_with_key — do a cmpxchg, but like, respectfully (it honors storage keys) ✨

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🦋 SYNOPSIS

int __cmpxchg_with_key (union kvm_s390_quad *ptr , union kvm_s390_quad
*old , union kvm_s390_quad new , int size , u8 access_key );

yes, that's the whole signature, copy it exactly, don't get creative with it 💖

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🎀 ARGUMENTS

• ptr — this is the address of the value you wanna compare against *old and then exchange with new. heads up though: it must be aligned to size — that's not a vibe suggestion, that's a requirement 🌸
• old — the old value, bestie. it gets compared to whatever's sitting at ptr to figure out whether the exchange actually happens. and plot twist: the old value that gets read from *ptr is written back here. so she's doing double duty 👑
• new — the new value you want to place at *ptr. the main character of the operation, honestly 🦋
• size — the size of the operation, in bytes. and here's the thing: it may only be a power of two up to 16. no weird numbers, no in-between sizes, we're keeping it clean ✨
• access_key — the access key used for checking storage key protection. this one's important, remember her 🎀

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💖 DESCRIPTION

so what she does is: perform a cmpxchg on guest memory, honoring storage key protection. iconic behaviour, honestly 🫶

and here's the detail you cannot skip: access_key alone determines how key checking is performed — neither storage-protection-override nor fetch-protection-override apply. periodt. 💅

⚠️ also important, don't miss this: in case of an exception *uval is set to zero. that's the rule, fr fr.

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🔥 RETURN

• 0 — cmpxchg executed successfully 😌 slay
• 1 — cmpxchg executed unsuccessfully 💀 it happens
• PGM_PROTECTION — an exception happened when trying to access *ptr 🙈
• -EAGAIN — maxed out number of retries (byte and short only — yes, only those two!)
• -EINVAL — invalid value for size 😭 check your powers of two, bestie

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🌈 SEE ALSO

kernel file ./arch/s390/kvm/s390/gaccess.c, plus the whole squad:

• guest_translate_gva(9)
• guest_range_to_gpas(9)
• cmpxchg_guest_abs_with_key(9)
• guest_translate_address_with_key(9)
• check_gva_range(9)
• check_gpa_range(9)
• kvm_s390_check_low_addr_prot_real(9)
• walk_guest_tables(9)
• __gaccess_shadow_fault(9)

iykyk 🧚

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🍓 okay that's the whole page!! this beauty lives over in ./arch/s390/kvm/s390 and the docs are dated October 2026 — so yeah, she's from the future, main character energy only 👑✨💜