[PATCH] KVM: x86: fix KVM_SET_CLOCK relative to setting correct clock value

[Marcelo Tosatti <mtosatti@redhat.com>]

In the masterclock enabled case, kvmclock_offset must be adjusted so
that user_ns.clock = master_kernel_ns + kvmclock_offset (that is, the
value set from KVM_SET_CLOCK is the one visible at system_timestamp).

This way the guest clock:

	1. Starts counting when KVM_SET_CLOCK executes.
	2. With the value provided by userspace.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

---
 arch/x86/kvm/x86.c |   23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

Index: kvm/arch/x86/kvm/x86.c
===================================================================
--- kvm.orig/arch/x86/kvm/x86.c	2017-04-27 17:37:48.131348255 -0300
+++ kvm/arch/x86/kvm/x86.c	2017-04-27 17:56:58.397530444 -0300
@@ -4172,8 +4172,9 @@
 		break;
 	}
 	case KVM_SET_CLOCK: {
-		struct kvm_clock_data user_ns;
 		u64 now_ns;
+		struct kvm_clock_data user_ns;
+		struct kvm_arch *ka = &kvm->arch;
 
 		r = -EFAULT;
 		if (copy_from_user(&user_ns, argp, sizeof(user_ns)))
@@ -4184,9 +4185,25 @@
 			goto out;
 
 		r = 0;
-		now_ns = get_kvmclock_ns(kvm);
-		kvm->arch.kvmclock_offset += user_ns.clock - now_ns;
+
 		kvm_gen_update_masterclock(kvm);
+		if (ka->use_master_clock) {
+			/*
+			 * In the masterclock enabled case,
+			 * kvmclock_offset must be adjusted so that
+			 * user_ns.clock = master_kernel_ns + kvmclock_offset
+			 * (that is, the value set from KVM_SET_CLOCK is the
+			 * one visible at system_timestamp).
+			 */
+			kvm->arch.kvmclock_offset = user_ns.clock -
+							ka->master_kernel_ns;
+
+			kvm_for_each_vcpu(i, vcpu, kvm)
+				kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
+		} else {
+			now_ns = get_kvmclock_ns(kvm);
+			kvm->arch.kvmclock_offset += user_ns.clock - now_ns;
+		}
 		break;
 	}
 	case KVM_GET_CLOCK: {