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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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