[PATCH kernel 1/3] x86/amd/dr_addr_mask: Cache values in percpu variables

[Alexey Kardashevskiy]

Reading DR[0-3]_ADDR_MASK MSRs takes about 250 cycles which is going to
be noticeable when the AMD KVM SEV-ES's DebugSwap feature is enabled and
KVM needs to store these before switching to a guest; the DebugSwitch
hardware support restores them as type B swap.

This stores MSR values from set_dr_addr_mask() in percpu values and
returns them via new get_dr_addr_mask(). The gain here is about 10x.

Signed-off-by: Alexey Kardashevskiy <aik@xxxxxxx>
---
 arch/x86/include/asm/debugreg.h |  1 +
 arch/x86/kernel/cpu/amd.c       | 32 ++++++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/arch/x86/include/asm/debugreg.h b/arch/x86/include/asm/debugreg.h
index cfdf307ddc01..c4324d0205b5 100644
--- a/arch/x86/include/asm/debugreg.h
+++ b/arch/x86/include/asm/debugreg.h
@@ -127,6 +127,7 @@ static __always_inline void local_db_restore(unsigned long dr7)
 
 #ifdef CONFIG_CPU_SUP_AMD
 extern void set_dr_addr_mask(unsigned long mask, int dr);
+extern unsigned long get_dr_addr_mask(int dr);
 #else
 static inline void set_dr_addr_mask(unsigned long mask, int dr) { }
 #endif
diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index c75d75b9f11a..ec7efcef4e14 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -1158,6 +1158,11 @@ static bool cpu_has_amd_erratum(struct cpuinfo_x86 *cpu, const int *erratum)
 	return false;
 }
 
+DEFINE_PER_CPU_READ_MOSTLY(unsigned long, dr0_addr_mask);
+DEFINE_PER_CPU_READ_MOSTLY(unsigned long, dr1_addr_mask);
+DEFINE_PER_CPU_READ_MOSTLY(unsigned long, dr2_addr_mask);
+DEFINE_PER_CPU_READ_MOSTLY(unsigned long, dr3_addr_mask);
+
 void set_dr_addr_mask(unsigned long mask, int dr)
 {
 	if (!boot_cpu_has(X86_FEATURE_BPEXT))
@@ -1166,17 +1171,44 @@ void set_dr_addr_mask(unsigned long mask, int dr)
 	switch (dr) {
 	case 0:
 		wrmsr(MSR_F16H_DR0_ADDR_MASK, mask, 0);
+		per_cpu(dr0_addr_mask, smp_processor_id()) = mask;
 		break;
 	case 1:
+		wrmsr(MSR_F16H_DR1_ADDR_MASK - 1 + dr, mask, 0);
+		per_cpu(dr1_addr_mask, smp_processor_id()) = mask;
+		break;
 	case 2:
+		wrmsr(MSR_F16H_DR1_ADDR_MASK - 1 + dr, mask, 0);
+		per_cpu(dr2_addr_mask, smp_processor_id()) = mask;
+		break;
 	case 3:
 		wrmsr(MSR_F16H_DR1_ADDR_MASK - 1 + dr, mask, 0);
+		per_cpu(dr3_addr_mask, smp_processor_id()) = mask;
 		break;
 	default:
 		break;
 	}
 }
 
+unsigned long get_dr_addr_mask(int dr)
+{
+	if (!boot_cpu_has(X86_FEATURE_BPEXT))
+		return 0;
+
+	switch (dr) {
+	case 0:
+		return per_cpu(dr0_addr_mask, smp_processor_id());
+	case 1:
+		return per_cpu(dr1_addr_mask, smp_processor_id());
+	case 2:
+		return per_cpu(dr2_addr_mask, smp_processor_id());
+	case 3:
+		return per_cpu(dr3_addr_mask, smp_processor_id());
+	}
+	return 0;
+}
+EXPORT_SYMBOL_GPL(get_dr_addr_mask);
+
 u32 amd_get_highest_perf(void)
 {
 	struct cpuinfo_x86 *c = &boot_cpu_data;
-- 
2.38.1