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Subject[PATCH 0/3] static_call/x86: Handle clang's conditional tail calls
Erhard reported boot fails on this AMD machine when using clang and bisected it
to a commit introducing a few static_call()s. Turns out that when using clang
with -Os it it very likely to generate conditional tail calls like:

0000000000000350 <amd_pmu_add_event>:
350: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) 351: R_X86_64_NONE __fentry__-0x4
355: 48 83 bf 20 01 00 00 00 cmpq $0x0,0x120(%rdi)
35d: 0f 85 00 00 00 00 jne 363 <amd_pmu_add_event+0x13> 35f: R_X86_64_PLT32 __SCT__amd_pmu_branch_add-0x4
363: e9 00 00 00 00 jmp 368 <amd_pmu_add_event+0x18> 364: R_X86_64_PLT32 __x86_return_thunk-0x4

And our inline static_call() patching code can't deal with those and BUG
happens -- really early.

These patches borrow the kprobe Jcc emulation to implement text_poke_bp() Jcc
support, which is then used to teach inline static_call() about this form.

---
arch/x86/include/asm/text-patching.h | 31 ++++++++++++++++++
arch/x86/kernel/alternative.c | 62 +++++++++++++++++++++++++++---------
arch/x86/kernel/kprobes/core.c | 38 +++++-----------------
arch/x86/kernel/static_call.c | 50 +++++++++++++++++++++++++++--
4 files changed, 133 insertions(+), 48 deletions(-)

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