x86/speculation: Simplify indirect_branch_prediction_barrier()

Make it all a function which does the WRMSR instead of having a hairy
inline asm.

[dwmw2: export it, fix CONFIG_RETPOLINE issues]

Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: ak@linux.intel.com
Cc: dave.hansen@intel.com
Cc: karahmed@amazon.de
Cc: arjan@linux.intel.com
Cc: torvalds@linux-foundation.org
Cc: peterz@infradead.org
Cc: bp@alien8.de
Cc: pbonzini@redhat.com
Cc: tim.c.chen@linux.intel.com
Cc: gregkh@linux-foundation.org
Link: https://lkml.kernel.org/r/1517070274-12128-4-git-send-email-dwmw@amazon.co.uk

3 files changed, 13 insertions, 9 deletions

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index df4ececa6ebf..d15d471348b8 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -164,15 +164,10 @@ static inline void vmexit_fill_RSB(void)
 
 static inline void indirect_branch_prediction_barrier(void)
 {
-        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");
+        alternative_input("",
+                          "call __ibp_barrier",
+                          X86_FEATURE_USE_IBPB,
+                          ASM_NO_INPUT_CLOBBER("eax", "ecx", "edx", "memory"));
 }
 
 #endif /* __ASSEMBLY__ */
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 9c18da64daa9..881ca3b1d6d4 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -970,4 +970,7 @@ 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 32d8e6cdc09e..3bfb2b23d79c 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -311,3 +311,9 @@ 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);