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From: Zachary Amsden on 15 Jun 2010 03:40
Some AMD based machines can have TSC drift when in C1 HLT state because
despite attempting to scale the TSC increment when dividing down the
P-state, the processor may return to full P-state to service cache
probes. The TSC of halted CPUs can advance faster than that of running
CPUs as a result, causing unpredictable TSC drift.

We implement a recommended workaround, which is disabling C1 clock ramping.
---
arch/x86/kvm/x86.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
1 files changed, 45 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ef847ee..8e836e9 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -56,6 +56,11 @@
#include <asm/i387.h>
#include <asm/xcr.h>

+#ifdef CONFIG_K8_NB
+#include <linux/pci.h>
+#include <asm/k8.h>
+#endif
+
#define MAX_IO_MSRS 256
#define CR0_RESERVED_BITS \
(~(unsigned long)(X86_CR0_PE | X86_CR0_MP | X86_CR0_EM | X86_CR0_TS \
@@ -4287,10 +4292,43 @@ static struct notifier_block kvmclock_cpu_notifier_block = {
.priority = -INT_MAX
};

+static u8 disabled_c1_ramp = 0;
+
static void kvm_timer_init(void)
{
int cpu;

+ /*
+ * AMD processors can de-synchronize TSC on halt in C1 state, because
+ * processors in lower P state will have TSC scaled properly during
+ * normal operation, but will have TSC scaled improperly while
+ * servicing cache probes. Because there is no way to determine how
+ * TSC was adjusted during cache probes, there are two solutions:
+ * resynchronize after halt, or disable C1-clock ramping.
+ *
+ * We implemenent solution 2.
+ */
+#ifdef CONFIG_K8_NB
+ if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD &&
+ boot_cpu_data.x86 == 0x0f &&
+ !boot_cpu_has(X86_FEATURE_CONSTANT_TSC)) {
+ struct pci_dev *nb;
+ int i;
+ cache_k8_northbridges();
+ for (i = 0; i < num_k8_northbridges; i++) {
+ u8 byte;
+ nb = k8_northbridges[i];
+ pci_read_config_byte(nb, 0x87, &byte);
+ if (byte & 1) {
+ printk(KERN_INFO "%s: AMD C1 clock ramping detected, performing workaround\n", __func__);
+ disabled_c1_ramp = byte;
+ pci_write_config_byte(nb, 0x87, byte & 0xFC);
+
+ }
+ }
+ }
+#endif
+
register_hotcpu_notifier(&kvmclock_cpu_notifier_block);
if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC)) {
cpufreq_register_notifier(&kvmclock_cpufreq_notifier_block,
@@ -4402,6 +4440,13 @@ void kvm_arch_exit(void)
unregister_hotcpu_notifier(&kvmclock_cpu_notifier_block);
kvm_x86_ops = NULL;
kvm_mmu_module_exit();
+#ifdef CONFIG_K8_NB
+ if (disabled_c1_ramp) {
+ struct pci_dev **nb;
+ for (nb = k8_northbridges; *nb; nb++)
+ pci_write_config_byte(*nb, 0x87, disabled_c1_ramp);
+ }
+#endif
}

int kvm_emulate_halt(struct kvm_vcpu *vcpu)
--
1.7.1

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Prev: [PATCH 08/17] Add clock sync request to hardware enable
Next: [PATCH 16/17] TSC reset compensation