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authorGlauber Costa <glommer@redhat.com>2010-02-01 16:54:05 -0200
committerGreg Kroah-Hartman <gregkh@suse.de>2010-02-09 04:50:56 -0800
commit4f7d6662c57dbaa6be09cc0bad2c01d005638a4d (patch)
treeb0ad11a70fc0aae87001c29e1996156ab5e2526e /Documentation
parenta74e62c2ef1fda92ad697556261b0e00fee5d581 (diff)
KVM: allow userspace to adjust kvmclock offset
(cherry picked from afbcf7ab8d1bc8c2d04792f6d9e786e0adeb328d) When we migrate a kvm guest that uses pvclock between two hosts, we may suffer a large skew. This is because there can be significant differences between the monotonic clock of the hosts involved. When a new host with a much larger monotonic time starts running the guest, the view of time will be significantly impacted. Situation is much worse when we do the opposite, and migrate to a host with a smaller monotonic clock. This proposed ioctl will allow userspace to inform us what is the monotonic clock value in the source host, so we can keep the time skew short, and more importantly, never goes backwards. Userspace may also need to trigger the current data, since from the first migration onwards, it won't be reflected by a simple call to clock_gettime() anymore. [marcelo: future-proof abi with a flags field] [jan: fix KVM_GET_CLOCK by clearing flags field instead of checking it] Signed-off-by: Glauber Costa <glommer@redhat.com> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com> Signed-off-by: Avi Kivity <avi@redhat.com> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/kvm/api.txt36
1 files changed, 36 insertions, 0 deletions
diff --git a/Documentation/kvm/api.txt b/Documentation/kvm/api.txt
index 5a4bc8cf6d04..db3a7062d793 100644
--- a/Documentation/kvm/api.txt
+++ b/Documentation/kvm/api.txt
@@ -593,6 +593,42 @@ struct kvm_irqchip {
} chip;
};
+4.27 KVM_GET_CLOCK
+
+Capability: KVM_CAP_ADJUST_CLOCK
+Architectures: x86
+Type: vm ioctl
+Parameters: struct kvm_clock_data (out)
+Returns: 0 on success, -1 on error
+
+Gets the current timestamp of kvmclock as seen by the current guest. In
+conjunction with KVM_SET_CLOCK, it is used to ensure monotonicity on scenarios
+such as migration.
+
+struct kvm_clock_data {
+ __u64 clock; /* kvmclock current value */
+ __u32 flags;
+ __u32 pad[9];
+};
+
+4.28 KVM_SET_CLOCK
+
+Capability: KVM_CAP_ADJUST_CLOCK
+Architectures: x86
+Type: vm ioctl
+Parameters: struct kvm_clock_data (in)
+Returns: 0 on success, -1 on error
+
+Sets the current timestamp of kvmclock to the valued specific in its parameter.
+In conjunction with KVM_GET_CLOCK, it is used to ensure monotonicity on scenarios
+such as migration.
+
+struct kvm_clock_data {
+ __u64 clock; /* kvmclock current value */
+ __u32 flags;
+ __u32 pad[9];
+};
+
5. The kvm_run structure
Application code obtains a pointer to the kvm_run structure by