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Subject[PATCH 26/35] x86: KVM guest: disable clock before rebooting.
Date
From: Glauber Costa <gcosta@redhat.com>

This patch writes 0 (actually, what really matters is that the
LSB is cleared) to the system time msr before shutting down
the machine for kexec.

Without it, we can have a random memory location being written
when the guest comes back

It overrides the functions shutdown, used in the path of kernel_kexec() (sys.c)
and crash_shutdown, used in the path of crash_kexec() (kexec.c)

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>
---
arch/x86/kernel/kvmclock.c | 27 +++++++++++++++++++++++++++
1 files changed, 27 insertions(+), 0 deletions(-)
diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index b999f5e..ddee040 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -22,6 +22,7 @@
#include <asm/msr.h>
#include <asm/apic.h>
#include <linux/percpu.h>
+#include <asm/reboot.h>

#define KVM_SCALE 22

@@ -143,6 +144,28 @@ static void kvm_setup_secondary_clock(void)
setup_secondary_APIC_clock();
}

+/*
+ * After the clock is registered, the host will keep writing to the
+ * registered memory location. If the guest happens to shutdown, this memory
+ * won't be valid. In cases like kexec, in which you install a new kernel, this
+ * means a random memory location will be kept being written. So before any
+ * kind of shutdown from our side, we unregister the clock by writting anything
+ * that does not have the 'enable' bit set in the msr
+ */
+#ifdef CONFIG_KEXEC
+static void kvm_crash_shutdown(struct pt_regs *regs)
+{
+ native_write_msr_safe(MSR_KVM_SYSTEM_TIME, 0, 0);
+ native_machine_crash_shutdown(regs);
+}
+#endif
+
+static void kvm_shutdown(void)
+{
+ native_write_msr_safe(MSR_KVM_SYSTEM_TIME, 0, 0);
+ native_machine_shutdown();
+}
+
void __init kvmclock_init(void)
{
if (!kvm_para_available())
@@ -155,6 +178,10 @@ void __init kvmclock_init(void)
pv_time_ops.set_wallclock = kvm_set_wallclock;
pv_time_ops.sched_clock = kvm_clock_read;
pv_apic_ops.setup_secondary_clock = kvm_setup_secondary_clock;
+ machine_ops.shutdown = kvm_shutdown;
+#ifdef CONFIG_KEXEC
+ machine_ops.crash_shutdown = kvm_crash_shutdown;
+#endif
clocksource_register(&kvm_clock);
}
}
--
1.5.4.5


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