[PATCH] v4: allow userspace to adjust kvmclock offset

[Glauber Costa]

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.

[ v2: uses a struct with a padding ]
[ v3: provide an ioctl to get clock data too ]
[ v4: used fixed-width signed type for delta ]

Signed-off-by: Glauber Costa <glommer@xxxxxxxxxx>
---
 arch/x86/include/asm/kvm_host.h |    1 +
 arch/x86/kvm/x86.c              |   35 ++++++++++++++++++++++++++++++++++-
 include/linux/kvm.h             |    7 +++++++
 3 files changed, 42 insertions(+), 1 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 179a919..c9b0d9f 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -410,6 +410,7 @@ struct kvm_arch{
 
 	unsigned long irq_sources_bitmap;
 	u64 vm_init_tsc;
+	s64 kvmclock_offset;
 };
 
 struct kvm_vm_stat {
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 9601bc6..09f31e2 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -699,7 +699,8 @@ static void kvm_write_guest_time(struct kvm_vcpu *v)
 	/* With all the info we got, fill in the values */
 
 	vcpu->hv_clock.system_time = ts.tv_nsec +
-				     (NSEC_PER_SEC * (u64)ts.tv_sec);
+				     (NSEC_PER_SEC * (u64)ts.tv_sec) + v->kvm->arch.kvmclock_offset;
+
 	/*
 	 * The interface expects us to write an even number signaling that the
 	 * update is finished. Since the guest won't see the intermediate
@@ -2441,6 +2442,38 @@ long kvm_arch_vm_ioctl(struct file *filp,
 		r = 0;
 		break;
 	}
+	case KVM_SET_CLOCK: {
+		struct timespec now;
+		struct kvm_clock_data user_ns;
+		u64 now_ns;
+		s64 delta;
+
+		r =  -EFAULT;
+		if (copy_from_user(&user_ns, argp, sizeof(user_ns)))
+			goto out;
+
+		r = 0;
+		ktime_get_ts(&now);
+		now_ns = timespec_to_ns(&now);
+		delta = user_ns.clock - now_ns;
+		kvm->arch.kvmclock_offset = delta;
+		break;	
+	}
+	case KVM_GET_CLOCK: {
+		struct timespec now;
+		struct kvm_clock_data user_ns;
+		u64 now_ns;
+
+		ktime_get_ts(&now);
+		now_ns = timespec_to_ns(&now);
+		user_ns.clock = kvm->arch.kvmclock_offset + now_ns;
+
+		if (copy_to_user(argp, &user_ns, sizeof(user_ns)))
+			r =  -EFAULT;
+
+		break;	
+	}
+
 	default:
 		;
 	}
diff --git a/include/linux/kvm.h b/include/linux/kvm.h
index f8f8900..ad0ecbc 100644
--- a/include/linux/kvm.h
+++ b/include/linux/kvm.h
@@ -497,6 +497,11 @@ struct kvm_irqfd {
 	__u8  pad[20];
 };
 
+struct kvm_clock_data {
+	__u64 clock;
+	__u64 pad[2];
+};
+
 /*
  * ioctls for VM fds
  */
@@ -546,6 +551,8 @@ struct kvm_irqfd {
 #define KVM_CREATE_PIT2		   _IOW(KVMIO, 0x77, struct kvm_pit_config)
 #define KVM_SET_BOOT_CPU_ID        _IO(KVMIO, 0x78)
 #define KVM_IOEVENTFD             _IOW(KVMIO, 0x79, struct kvm_ioeventfd)
+#define KVM_SET_CLOCK		  _IOW(KVMIO, 0x7a, struct kvm_clock_data)
+#define KVM_GET_CLOCK		  _IOW(KVMIO, 0x7b, struct kvm_clock_data)
 
 /*
  * ioctls for vcpu fds
-- 
1.6.2.2

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