[PATCH 5.4 13/51] x86/bugs: Optimize SPEC_CTRL MSR writes

[Greg Kroah-Hartman]

From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

commit c779bc1a9002fa474175b80e72b85c9bf628abb0 upstream.

When changing SPEC_CTRL for user control, the WRMSR can be delayed
until return-to-user when KERNEL_IBRS has been enabled.

This avoids an MSR write during context switch.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Reviewed-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 arch/x86/include/asm/nospec-branch.h |    2 +-
 arch/x86/kernel/cpu/bugs.c           |   18 ++++++++++++------
 arch/x86/kernel/process.c            |    2 +-
 3 files changed, 14 insertions(+), 8 deletions(-)

--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -297,7 +297,7 @@ static inline void indirect_branch_predi
 
 /* The Intel SPEC CTRL MSR base value cache */
 extern u64 x86_spec_ctrl_base;
-extern void write_spec_ctrl_current(u64 val);
+extern void write_spec_ctrl_current(u64 val, bool force);
 
 /*
  * With retpoline, we must use IBRS to restrict branch prediction
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -61,13 +61,19 @@ static DEFINE_MUTEX(spec_ctrl_mutex);
  * Keep track of the SPEC_CTRL MSR value for the current task, which may differ
  * from x86_spec_ctrl_base due to STIBP/SSB in __speculation_ctrl_update().
  */
-void write_spec_ctrl_current(u64 val)
+void write_spec_ctrl_current(u64 val, bool force)
 {
 	if (this_cpu_read(x86_spec_ctrl_current) == val)
 		return;
 
 	this_cpu_write(x86_spec_ctrl_current, val);
-	wrmsrl(MSR_IA32_SPEC_CTRL, val);
+
+	/*
+	 * When KERNEL_IBRS this MSR is written on return-to-user, unless
+	 * forced the update can be delayed until that time.
+	 */
+	if (force || !cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS))
+		wrmsrl(MSR_IA32_SPEC_CTRL, val);
 }
 
 /*
@@ -1195,7 +1201,7 @@ static void __init spectre_v2_select_mit
 	if (spectre_v2_in_eibrs_mode(mode)) {
 		/* Force it so VMEXIT will restore correctly */
 		x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
-		write_spec_ctrl_current(x86_spec_ctrl_base);
+		write_spec_ctrl_current(x86_spec_ctrl_base, true);
 	}
 
 	switch (mode) {
@@ -1250,7 +1256,7 @@ static void __init spectre_v2_select_mit
 
 static void update_stibp_msr(void * __unused)
 {
-	write_spec_ctrl_current(x86_spec_ctrl_base);
+	write_spec_ctrl_current(x86_spec_ctrl_base, true);
 }
 
 /* Update x86_spec_ctrl_base in case SMT state changed. */
@@ -1493,7 +1499,7 @@ static enum ssb_mitigation __init __ssb_
 			x86_amd_ssb_disable();
 		} else {
 			x86_spec_ctrl_base |= SPEC_CTRL_SSBD;
-			write_spec_ctrl_current(x86_spec_ctrl_base);
+			write_spec_ctrl_current(x86_spec_ctrl_base, true);
 		}
 	}
 
@@ -1710,7 +1716,7 @@ int arch_prctl_spec_ctrl_get(struct task
 void x86_spec_ctrl_setup_ap(void)
 {
 	if (boot_cpu_has(X86_FEATURE_MSR_SPEC_CTRL))
-		write_spec_ctrl_current(x86_spec_ctrl_base);
+		write_spec_ctrl_current(x86_spec_ctrl_base, true);
 
 	if (ssb_mode == SPEC_STORE_BYPASS_DISABLE)
 		x86_amd_ssb_disable();
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -449,7 +449,7 @@ static __always_inline void __speculatio
 	}
 
 	if (updmsr)
-		write_spec_ctrl_current(msr);
+		write_spec_ctrl_current(msr, false);
 }
 
 static unsigned long speculation_ctrl_update_tif(struct task_struct *tsk)