[PATCH v2 2/2] turn off kvmclock when resetting cpu

[Glauber Costa <glommer@redhat.com>]

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 writting 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.

It is also necessary to reset other msrs, so this patch resets
everything that kvm exports through its MSR list.

This patch fixes the issue for me.

Signed-off-by: Glauber Costa <glommer@redhat.com>
---
 qemu-kvm-x86.c |   23 +++++++++++++++++++++++
 1 files changed, 23 insertions(+), 0 deletions(-)

diff --git a/qemu-kvm-x86.c b/qemu-kvm-x86.c
index 5a2b552..bd74316 100644
--- a/qemu-kvm-x86.c
+++ b/qemu-kvm-x86.c
@@ -1417,8 +1417,31 @@ void kvm_arch_push_nmi(void *opaque)
 }
 #endif /* KVM_CAP_USER_NMI */
 
+static int kvm_reset_msrs(CPUState *env)
+{
+    struct {
+        struct kvm_msrs info;
+        struct kvm_msr_entry entries[100];
+    } msr_data;
+    int n;
+    struct kvm_msr_entry *msrs = msr_data.entries;
+
+    if (!kvm_msr_list)
+        return -1;
+
+    for (n = 0; n < kvm_msr_list->nmsrs; n++) {
+        kvm_msr_entry_set(&msrs[n], kvm_msr_list->indices[n], 0);
+    }
+
+    msr_data.info.nmsrs = n;
+
+    return kvm_vcpu_ioctl(env, KVM_SET_MSRS, &msr_data);
+}
+
+
 void kvm_arch_cpu_reset(CPUState *env)
 {
+    kvm_reset_msrs(env);
     kvm_arch_reset_vcpu(env);
     kvm_reset_mpstate(env);
 }
-- 
1.6.2.2