Re: [PATCH] disable clock before rebooting.

[Avi Kivity <avi@qumranet.com>]

Glauber Costa wrote:
> This patch writes 0 (actually, what really matters is that the
> LSB is cleared) to the system time msr before rebooting/shutting down
> the machine.
>
> Without it, we can have a random memory location being written
> when the guest comes back
>
> Signed-off-by: Glauber Costa <gcosta@redhat.com>
> ---
>  arch/x86/kernel/kvmclock.c |   32 ++++++++++++++++++++++++++++++++
>  1 files changed, 32 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
> index f654a12..5c9ff8d 100644
> --- a/arch/x86/kernel/kvmclock.c
> +++ b/arch/x86/kernel/kvmclock.c
> @@ -21,6 +21,7 @@ #include <linux/kvm_para.h>
>  #include <asm/arch_hooks.h>
>  #include <asm/msr.h>
>  #include <linux/percpu.h>
> +#include <asm/reboot.h>
>  
>  #define KVM_SCALE 22
>  
> @@ -142,6 +143,32 @@ static void kvm_setup_secondary_clock(vo
>  	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, or restart,
> + * this memory won't be valid. In cases like kexec, in which you install a new kernel,
> + * this will mean 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
> + */ 
> +static void kvm_restart(char *unused) {
>   

This looks like a struct, with the { sitting there on the end.

> +	native_write_msr_safe(MSR_KVM_SYSTEM_TIME, 0, 0);
> +	native_machine_restart(unused);
> +}
> +
> +/* Forgive me dear lord, for my laziness */
> +#define kvm_reboot_fn(x) \
> +static void kvm_##x(void) { \
> +	native_write_msr_safe(MSR_KVM_SYSTEM_TIME, 0, 0); \
> +	native_machine_##x(); \
> +}
> +
> +kvm_reboot_fn(emergency_restart)
> +kvm_reboot_fn(shutdown)
> +kvm_reboot_fn(halt)
> +kvm_reboot_fn(power_off)
> +#undef kvm_reboot_fn
> +
>   

Why not go all the way and to _restart the same way?

-- 
Any sufficiently difficult bug is indistinguishable from a feature.


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