On 05/04/2010 09:35 PM, 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);
This fails if the kernel doesn't support those MSRs. Moreover, you need to use the new MSRs as well if we are ever to succeed in deprecating the old ones.
+ msr_data.info.nmsrs = n; + + return kvm_vcpu_ioctl(env, KVM_SET_MSRS,&msr_data); +} + +
How about a different approach? Query the supported MSRs (KVM_GET_MSR_LIST or thereabout) and reset them (with special cases for the TSC, and the old clock MSRs when the new ones are present)?
Long term we need a kernel reset function, but this will do for now. -- error compiling committee.c: too many arguments to function -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html