[Patch v2 3/4] x86/speculation: Extend per process STIBP to AMD cpus.

[Tim Chen <tim.c.chen@linux.intel.com>]

From: Thomas Lendacky <Thomas.Lendacky@amd.com>

We extend the app to app spectre v2 mitigation using STIBP
to the AMD cpus. We need to take care of special
cases for AMD cpu's update of SPEC_CTRL MSR to avoid double
writing of MSRs from update to SSBD and STIBP.

Originally-by: Thomas Lendacky <Thomas.Lendacky@amd.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
---
 arch/x86/kernel/process.c | 48 +++++++++++++++++++++++++++++++++++++----------
 1 file changed, 38 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index cb24014..4a3a672 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -399,6 +399,10 @@ static __always_inline void set_spec_ctrl_state(unsigned long tifn)
 {
 	u64 msr = x86_spec_ctrl_base;
 
+	/*
+	 * AMD cpu may have used a different method to update SSBD, so
+	 * we need to be sure we are using the SPEC_CTRL MSR for SSBD.
+	 */
 	if (static_cpu_has(X86_FEATURE_SSBD))
 		msr |= ssbd_tif_to_spec_ctrl(tifn);
 
@@ -408,20 +412,45 @@ static __always_inline void set_spec_ctrl_state(unsigned long tifn)
 	wrmsrl(MSR_IA32_SPEC_CTRL, msr);
 }
 
-static __always_inline void __speculative_store_bypass_update(unsigned long tifn)
+static __always_inline void __speculative_store_bypass_update(unsigned long tifp,
+							      unsigned long tifn)
 {
-	if (static_cpu_has(X86_FEATURE_VIRT_SSBD))
-		amd_set_ssb_virt_state(tifn);
-	else if (static_cpu_has(X86_FEATURE_LS_CFG_SSBD))
-		amd_set_core_ssb_state(tifn);
-	else
-		set_spec_ctrl_state(tifn);
+	bool stibp = !!((tifp ^ tifn) & _TIF_STIBP);
+	bool ssbd = !!((tifp ^ tifn) & _TIF_SSBD);
+
+	if (!ssbd && !stibp)
+		return;
+
+	if (ssbd) {
+		/*
+		 * For AMD, try these methods first.  The ssbd variable will
+		 * reflect if the SPEC_CTRL MSR method is needed.
+		 */
+		ssbd = false;
+
+		if (static_cpu_has(X86_FEATURE_VIRT_SSBD))
+			amd_set_ssb_virt_state(tifn);
+		else if (static_cpu_has(X86_FEATURE_LS_CFG_SSBD))
+			amd_set_core_ssb_state(tifn);
+		else
+			ssbd = true;
+	}
+
+	/* Avoid a possible extra MSR write, recheck the flags */
+	if (!ssbd && !stibp)
+		return;
+
+	set_spec_ctrl_state(tifn);
 }
 
 void speculative_store_bypass_update(unsigned long tif)
 {
+	/*
+	 * On this path we're forcing the update, so use ~tif as the
+	 * previous flags.
+	 */
 	preempt_disable();
-	__speculative_store_bypass_update(tif);
+	__speculative_store_bypass_update(~tif, tif);
 	preempt_enable();
 }
 
@@ -457,8 +486,7 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
 	if ((tifp ^ tifn) & _TIF_NOCPUID)
 		set_cpuid_faulting(!!(tifn & _TIF_NOCPUID));
 
-	if ((tifp ^ tifn) & (_TIF_SSBD | _TIF_STIBP))
-		__speculative_store_bypass_update(tifn);
+	__speculative_store_bypass_update(tifp, tifn);
 }
 
 /*
-- 
2.9.4