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Subject[RFC PATCH 1/4] Revert "x86/speculation: Simplify indirect_branch_prediction_barrier()"
Date
This reverts commit 64e16720ea0879f8ab4547e3b9758936d483909b.

We cannot call C functions like that, without marking all the
call-clobbered registers as, well, clobbered. We might have got away
with it for now because the __ibp_barrier() function was *fairly*
unlikely to actually use any other registers. But no. Just no.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
arch/x86/include/asm/nospec-branch.h | 13 +++++++++----
arch/x86/include/asm/processor.h | 3 ---
arch/x86/kernel/cpu/bugs.c | 6 ------
3 files changed, 9 insertions(+), 13 deletions(-)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 4d57894..300cc15 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -164,10 +164,15 @@ static inline void vmexit_fill_RSB(void)

static inline void indirect_branch_prediction_barrier(void)
{
- alternative_input("",
- "call __ibp_barrier",
- X86_FEATURE_USE_IBPB,
- ASM_NO_INPUT_CLOBBER("eax", "ecx", "edx", "memory"));
+ asm volatile(ALTERNATIVE("",
+ "movl %[msr], %%ecx\n\t"
+ "movl %[val], %%eax\n\t"
+ "movl $0, %%edx\n\t"
+ "wrmsr",
+ X86_FEATURE_USE_IBPB)
+ : : [msr] "i" (MSR_IA32_PRED_CMD),
+ [val] "i" (PRED_CMD_IBPB)
+ : "eax", "ecx", "edx", "memory");
}

#endif /* __ASSEMBLY__ */
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 513f960..99799fb 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -969,7 +969,4 @@ bool xen_set_default_idle(void);

void stop_this_cpu(void *dummy);
void df_debug(struct pt_regs *regs, long error_code);
-
-void __ibp_barrier(void);
-
#endif /* _ASM_X86_PROCESSOR_H */
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 71949bf..61152aa 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -337,9 +337,3 @@ ssize_t cpu_show_spectre_v2(struct device *dev,
spectre_v2_module_string());
}
#endif
-
-void __ibp_barrier(void)
-{
- __wrmsr(MSR_IA32_PRED_CMD, PRED_CMD_IBPB, 0);
-}
-EXPORT_SYMBOL_GPL(__ibp_barrier);
--
2.7.4
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