[RFC 06/10] x86/speculation: Add inlines to control Indirect Branch Speculation

[KarimAllah Ahmed]

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

XX: I am utterly unconvinced that having "friendly, self-explanatory"
    names for the IBRS-frobbing inlines is useful. There be dragons
    here for anyone who isn't intimately familiar with what's going
    on, and it's almost better to just call it IBRS, put a reference
    to the spec, and have a clear "you must be âthisâ tall to ride."

[karahmed: switch to using ALTERNATIVES instead of static_cpu_has]
[dwmw2: wrmsr args inside the ALTERNATIVE again, bikeshed naming]

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: KarimAllah Ahmed <karahmed@xxxxxxxxx>
Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
---
 arch/x86/include/asm/nospec-branch.h | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 8759449..5be3443 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -232,5 +232,41 @@ static inline void indirect_branch_prediction_barrier(void)
 		     : "memory");
 }
 
+/*
+ * This also performs a barrier, and setting it again when it was already
+ * set is NOT a no-op.
+ */
+static inline void restrict_branch_speculation(void)
+{
+	unsigned long ax, cx, dx;
+
+	asm volatile(ALTERNATIVE("",
+				 "movl %[msr], %%ecx\n\t"
+				 "movl %[val], %%eax\n\t"
+				 "movl $0, %%edx\n\t"
+				 "wrmsr",
+				 X86_FEATURE_IBRS)
+		     : "=a" (ax), "=c" (cx), "=d" (dx)
+		     : [msr] "i" (MSR_IA32_SPEC_CTRL),
+		       [val] "i" (SPEC_CTRL_IBRS)
+		     : "memory");
+}
+
+static inline void unrestrict_branch_speculation(void)
+{
+	unsigned long ax, cx, dx;
+
+	asm volatile(ALTERNATIVE("",
+				 "movl %[msr], %%ecx\n\t"
+				 "movl %[val], %%eax\n\t"
+				 "movl $0, %%edx\n\t"
+				 "wrmsr",
+				 X86_FEATURE_IBRS)
+		     : "=a" (ax), "=c" (cx), "=d" (dx)
+		     : [msr] "i" (MSR_IA32_SPEC_CTRL),
+		       [val] "i" (0)
+		     : "memory");
+}
+
 #endif /* __ASSEMBLY__ */
 #endif /* __NOSPEC_BRANCH_H__ */
-- 
2.7.4