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Subject[PATCH 3/5] Try using new kvm clock msrs
Date
We now added a new set of clock-related msrs in replacement of the old
ones. In theory, we could just try to use them and get a return value
indicating they do not exist, due to our use of kvm_write_msr_save.

However, kvm clock registration happens very early, and if we ever
try to write to a non-existant MSR, we raise a lethal #GP, since our
idt handlers are not in place yet.

So this patch tests for a cpuid feature exported by the host to
decide which set of msrs are supported.

Signed-off-by: Glauber Costa <glommer@redhat.com>
---
arch/x86/include/asm/kvm_para.h | 4 ++
arch/x86/kernel/kvmclock.c | 68 +++++++++++++++++++++++++++------------
2 files changed, 51 insertions(+), 21 deletions(-)
diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
index 0cffb96..a32710a 100644
--- a/arch/x86/include/asm/kvm_para.h
+++ b/arch/x86/include/asm/kvm_para.h
@@ -16,6 +16,10 @@
#define KVM_FEATURE_CLOCKSOURCE 0
#define KVM_FEATURE_NOP_IO_DELAY 1
#define KVM_FEATURE_MMU_OP 2
+/* We could just try to use new msr values, but they are queried very early,
+ * kernel does not have idt handlers yet, and failures are fatal */
+#define KVM_FEATURE_CLOCKSOURCE2 3
+

#define MSR_KVM_WALL_CLOCK_OLD 0x11
#define MSR_KVM_SYSTEM_TIME_OLD 0x12
diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index feaeb0d..6d814ce 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -29,6 +29,7 @@
#define KVM_SCALE 22

static int kvmclock = 1;
+static int kvm_use_new_msrs = 0;

static int parse_no_kvmclock(char *arg)
{
@@ -41,6 +42,18 @@ early_param("no-kvmclock", parse_no_kvmclock);
static DEFINE_PER_CPU_SHARED_ALIGNED(struct pvclock_vcpu_time_info, hv_clock);
static struct pvclock_wall_clock wall_clock;

+static int kvm_system_time_write_value(int low, int high)
+{
+ if (kvm_use_new_msrs)
+ return native_write_msr_safe(MSR_KVM_SYSTEM_TIME_OLD, low, high);
+ else
+ return native_write_msr_safe(MSR_KVM_SYSTEM_TIME, low, high);
+}
+
+static void kvm_turnoff_clock(void)
+{
+ kvm_system_time_write_value(0, 0);
+}
/*
* The wallclock is the time of day when we booted. Since then, some time may
* have elapsed since the hypervisor wrote the data. So we try to account for
@@ -54,7 +67,11 @@ static unsigned long kvm_get_wallclock(void)

low = (int)__pa_symbol(&wall_clock);
high = ((u64)__pa_symbol(&wall_clock) >> 32);
- native_write_msr(MSR_KVM_WALL_CLOCK, low, high);
+
+ if (kvm_use_new_msrs)
+ native_write_msr_safe(MSR_KVM_WALL_CLOCK, low, high);
+ else
+ native_write_msr(MSR_KVM_WALL_CLOCK_OLD, low, high);

vcpu_time = &get_cpu_var(hv_clock);
pvclock_read_wallclock(&wall_clock, vcpu_time, &ts);
@@ -130,7 +147,8 @@ static int kvm_register_clock(char *txt)
high = ((u64)__pa(&per_cpu(hv_clock, cpu)) >> 32);
printk(KERN_INFO "kvm-clock: cpu %d, msr %x:%x, %s\n",
cpu, high, low, txt);
- return native_write_msr_safe(MSR_KVM_SYSTEM_TIME, low, high);
+
+ return kvm_system_time_write_value(low, high);
}

#ifdef CONFIG_X86_LOCAL_APIC
@@ -165,14 +183,14 @@ static void __init kvm_smp_prepare_boot_cpu(void)
#ifdef CONFIG_KEXEC
static void kvm_crash_shutdown(struct pt_regs *regs)
{
- native_write_msr_safe(MSR_KVM_SYSTEM_TIME, 0, 0);
+ kvm_turnoff_clock();
native_machine_crash_shutdown(regs);
}
#endif

static void kvm_shutdown(void)
{
- native_write_msr_safe(MSR_KVM_SYSTEM_TIME, 0, 0);
+ kvm_turnoff_clock();
native_machine_shutdown();
}

@@ -181,27 +199,35 @@ void __init kvmclock_init(void)
if (!kvm_para_available())
return;

- if (kvmclock && kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE)) {
- if (kvm_register_clock("boot clock"))
- return;
- pv_time_ops.sched_clock = kvm_clock_read;
- x86_platform.calibrate_tsc = kvm_get_tsc_khz;
- x86_platform.get_wallclock = kvm_get_wallclock;
- x86_platform.set_wallclock = kvm_set_wallclock;
+ if (kvmclock && kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE2))
+ kvm_use_new_msrs = 1;
+ else if (kvmclock && kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE))
+ kvm_use_new_msrs = 0;
+ else
+ return;
+
+ printk(KERN_INFO "kvm-clock: %ssing clocksource new msrs",
+ kvm_use_new_msrs ? "U": "Not u");
+
+ if (kvm_register_clock("boot clock"))
+ return;
+ pv_time_ops.sched_clock = kvm_clock_read;
+ x86_platform.calibrate_tsc = kvm_get_tsc_khz;
+ x86_platform.get_wallclock = kvm_get_wallclock;
+ x86_platform.set_wallclock = kvm_set_wallclock;
#ifdef CONFIG_X86_LOCAL_APIC
- x86_cpuinit.setup_percpu_clockev =
- kvm_setup_secondary_clock;
+ x86_cpuinit.setup_percpu_clockev =
+ kvm_setup_secondary_clock;
#endif
#ifdef CONFIG_SMP
- smp_ops.smp_prepare_boot_cpu = kvm_smp_prepare_boot_cpu;
+ smp_ops.smp_prepare_boot_cpu = kvm_smp_prepare_boot_cpu;
#endif
- machine_ops.shutdown = kvm_shutdown;
+ machine_ops.shutdown = kvm_shutdown;
#ifdef CONFIG_KEXEC
- machine_ops.crash_shutdown = kvm_crash_shutdown;
+ machine_ops.crash_shutdown = kvm_crash_shutdown;
#endif
- kvm_get_preset_lpj();
- clocksource_register(&kvm_clock);
- pv_info.paravirt_enabled = 1;
- pv_info.name = "KVM";
- }
+ kvm_get_preset_lpj();
+ clocksource_register(&kvm_clock);
+ pv_info.paravirt_enabled = 1;
+ pv_info.name = "KVM";
}
--
1.6.2.2


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