On Tue, May 04, 2010 at 02:35:28PM -0400, Glauber Costa wrote:
> Currently, in the linux kernel, we reset kvmclock if we are rebooting
> into a crash kernel through kexec. The rationale, is that a new kernel
> won't follow the same memory addresses, and the memory where kvmclock is
> located in the first kernel, will be something else in the second one.
>
> We don't do it in normal reboots, because the second kernel ends up
> registering kvmclock again, which has the effect of turning off the
> first instance.
>
> This is, however, totally wrong. This assumes we're booting into
> a kernel that also has kvmclock enabled. If by some reason we reboot
> into something that doesn't do kvmclock including but not limited to:
> * rebooting into an older kernel without kvmclock support,
> * rebooting with no-kvmclock,
> * rebootint into another O.S,
>
> we'll simply have the hypervisor writing into a random memory position
> into the guest. Neat, uh?
>
> Moreover, I believe the fix belongs in qemu, since it is the entity
> more prepared to detect all kinds of reboots (by means of a cpu_reset),
> not to mention the presence of misbehaving guests, that can forget
> to turn kvmclock off.
>
> This patch fixes the issue for me.
>
> Signed-off-by: Glauber Costa <glommer@xxxxxxxxxx>
> ---
> qemu-kvm-x86.c | 19 +++++++++++++++++++
> 1 files changed, 19 insertions(+), 0 deletions(-)
>
> diff --git a/qemu-kvm-x86.c b/qemu-kvm-x86.c
> index 439c31a..4b94e04 100644
> --- a/qemu-kvm-x86.c
> +++ b/qemu-kvm-x86.c
> @@ -1417,8 +1417,27 @@ void kvm_arch_push_nmi(void *opaque)
> }
> #endif /* KVM_CAP_USER_NMI */
>
> +static int kvm_turn_off_clock(CPUState *env)
> +{
> + struct {
> + struct kvm_msrs info;
> + struct kvm_msr_entry entries[100];
> + } msr_data;
> +
> + struct kvm_msr_entry *msrs = msr_data.entries;
> + int n = 0;
> +
> + kvm_msr_entry_set(&msrs[n++], MSR_KVM_SYSTEM_TIME, 0);
> + kvm_msr_entry_set(&msrs[n++], MSR_KVM_WALL_CLOCK, 0);
> + msr_data.info.nmsrs = n;
> +
> + return kvm_vcpu_ioctl(env, KVM_SET_MSRS, &msr_data);
> +}
> +
> +
> void kvm_arch_cpu_reset(CPUState *env)
> {
> + kvm_turn_off_clock(env);
Better to zero env->system_time_msr and wall_clock_msr here, and modify
kvm_arch_load_regs to write those MSRs on KVM_PUT_RESET_STATE (to be
conformant with the new writeback scheme).
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