KVM: x86: Expose TSC offset controls to userspace

To date, VMM-directed TSC synchronization and migration has been a bit
messy. KVM has some baked-in heuristics around TSC writes to infer if
the VMM is attempting to synchronize. This is problematic, as it depends
on host userspace writing to the guest's TSC within 1 second of the last
write.

A much cleaner approach to configuring the guest's views of the TSC is to
simply migrate the TSC offset for every vCPU. Offsets are idempotent,
and thus not subject to change depending on when the VMM actually
reads/writes values from/to KVM. The VMM can then read the TSC once with
KVM_GET_CLOCK to capture a (realtime, host_tsc) pair at the instant when
the guest is paused.

Cc: David Matlack <[e-mail]>
Cc: Sean Christopherson <[e-mail]>
Signed-off-by: Oliver Upton <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>

Documentation/virt/kvm/devices/vcpu.rst

···
 base address must be 64 byte aligned and exist within a valid guest memory
 region. See Documentation/virt/kvm/arm/pvtime.rst for more information
 including the layout of the stolen time structure.
+
+4. GROUP: KVM_VCPU_TSC_CTRL
+===========================
+
+:Architectures: x86
+
+4.1 ATTRIBUTE: KVM_VCPU_TSC_OFFSET
+
+:Parameters: 64-bit unsigned TSC offset
+
+Returns:
+
+	 ======= ======================================
+	 -EFAULT Error reading/writing the provided
+		 parameter address.
+	 -ENXIO  Attribute not supported
+	 ======= ======================================
+
+Specifies the guest's TSC offset relative to the host's TSC. The guest's
+TSC is then derived by the following equation:
+
+  guest_tsc = host_tsc + KVM_VCPU_TSC_OFFSET
+
+This attribute is useful for the precise migration of a guest's TSC. The
+following describes a possible algorithm to use for the migration of a
+guest's TSC:
+
+From the source VMM process:
+
+1. Invoke the KVM_GET_CLOCK ioctl to record the host TSC (t_0),
+   kvmclock nanoseconds (k_0), and realtime nanoseconds (r_0).
+
+2. Read the KVM_VCPU_TSC_OFFSET attribute for every vCPU to record the
+   guest TSC offset (off_n).
+
+3. Invoke the KVM_GET_TSC_KHZ ioctl to record the frequency of the
+   guest's TSC (freq).
+
+From the destination VMM process:
+
+4. Invoke the KVM_SET_CLOCK ioctl, providing the kvmclock nanoseconds
+   (k_0) and realtime nanoseconds (r_0) in their respective fields.
+   Ensure that the KVM_CLOCK_REALTIME flag is set in the provided
+   structure. KVM will advance the VM's kvmclock to account for elapsed
+   time since recording the clock values.
+
+5. Invoke the KVM_GET_CLOCK ioctl to record the host TSC (t_1) and
+   kvmclock nanoseconds (k_1).
+
+6. Adjust the guest TSC offsets for every vCPU to account for (1) time
+   elapsed since recording state and (2) difference in TSCs between the
+   source and destination machine:
+
+   new_off_n = t_0 + off_n + (k_1 - k_0) * freq - t_1
+
+7. Write the KVM_VCPU_TSC_OFFSET attribute for every vCPU with the
+   respective value derived in the previous step.

arch/x86/include/asm/kvm_host.h

···
 	u64 last_tsc_nsec;
 	u64 last_tsc_write;
 	u32 last_tsc_khz;
+	u64 last_tsc_offset;
 	u64 cur_tsc_nsec;
 	u64 cur_tsc_write;
 	u64 cur_tsc_offset;

arch/x86/include/uapi/asm/kvm.h

···
 #define KVM_PMU_EVENT_ALLOW 0
 #define KVM_PMU_EVENT_DENY 1
 
+/* for KVM_{GET,SET,HAS}_DEVICE_ATTR */
+#define KVM_VCPU_TSC_CTRL 0 /* control group for the timestamp counter (TSC) */
+#define   KVM_VCPU_TSC_OFFSET 0 /* attribute for the TSC offset */
+
 #endif /* _ASM_X86_KVM_H */

arch/x86/kvm/x86.c

···
 	kvm->arch.last_tsc_nsec = ns;
 	kvm->arch.last_tsc_write = tsc;
 	kvm->arch.last_tsc_khz = vcpu->arch.virtual_tsc_khz;
+	kvm->arch.last_tsc_offset = offset;
 
 	vcpu->arch.last_guest_tsc = tsc;
 
···
 	case KVM_CAP_VM_COPY_ENC_CONTEXT_FROM:
 	case KVM_CAP_SREGS2:
 	case KVM_CAP_EXIT_ON_EMULATION_FAILURE:
+	case KVM_CAP_VCPU_ATTRIBUTES:
 		r = 1;
 		break;
 	case KVM_CAP_EXIT_HYPERCALL:
···
 	return 0;
 }
 
+static int kvm_arch_tsc_has_attr(struct kvm_vcpu *vcpu,
+				 struct kvm_device_attr *attr)
+{
+	int r;
+
+	switch (attr->attr) {
+	case KVM_VCPU_TSC_OFFSET:
+		r = 0;
+		break;
+	default:
+		r = -ENXIO;
+	}
+
+	return r;
+}
+
+static int kvm_arch_tsc_get_attr(struct kvm_vcpu *vcpu,
+				 struct kvm_device_attr *attr)
+{
+	u64 __user *uaddr = (u64 __user *)(unsigned long)attr->addr;
+	int r;
+
+	if ((u64)(unsigned long)uaddr != attr->addr)
+		return -EFAULT;
+
+	switch (attr->attr) {
+	case KVM_VCPU_TSC_OFFSET:
+		r = -EFAULT;
+		if (put_user(vcpu->arch.l1_tsc_offset, uaddr))
+			break;
+		r = 0;
+		break;
+	default:
+		r = -ENXIO;
+	}
+
+	return r;
+}
+
+static int kvm_arch_tsc_set_attr(struct kvm_vcpu *vcpu,
+				 struct kvm_device_attr *attr)
+{
+	u64 __user *uaddr = (u64 __user *)(unsigned long)attr->addr;
+	struct kvm *kvm = vcpu->kvm;
+	int r;
+
+	if ((u64)(unsigned long)uaddr != attr->addr)
+		return -EFAULT;
+
+	switch (attr->attr) {
+	case KVM_VCPU_TSC_OFFSET: {
+		u64 offset, tsc, ns;
+		unsigned long flags;
+		bool matched;
+
+		r = -EFAULT;
+		if (get_user(offset, uaddr))
+			break;
+
+		raw_spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
+
+		matched = (vcpu->arch.virtual_tsc_khz &&
+			   kvm->arch.last_tsc_khz == vcpu->arch.virtual_tsc_khz &&
+			   kvm->arch.last_tsc_offset == offset);
+
+		tsc = kvm_scale_tsc(vcpu, rdtsc(), vcpu->arch.l1_tsc_scaling_ratio) + offset;
+		ns = get_kvmclock_base_ns();
+
+		__kvm_synchronize_tsc(vcpu, offset, tsc, ns, matched);
+		raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
+
+		r = 0;
+		break;
+	}
+	default:
+		r = -ENXIO;
+	}
+
+	return r;
+}
+
+static int kvm_vcpu_ioctl_device_attr(struct kvm_vcpu *vcpu,
+				      unsigned int ioctl,
+				      void __user *argp)
+{
+	struct kvm_device_attr attr;
+	int r;
+
+	if (copy_from_user(&attr, argp, sizeof(attr)))
+		return -EFAULT;
+
+	if (attr.group != KVM_VCPU_TSC_CTRL)
+		return -ENXIO;
+
+	switch (ioctl) {
+	case KVM_HAS_DEVICE_ATTR:
+		r = kvm_arch_tsc_has_attr(vcpu, &attr);
+		break;
+	case KVM_GET_DEVICE_ATTR:
+		r = kvm_arch_tsc_get_attr(vcpu, &attr);
+		break;
+	case KVM_SET_DEVICE_ATTR:
+		r = kvm_arch_tsc_set_attr(vcpu, &attr);
+		break;
+	}
+
+	return r;
+}
+
 static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu,
 				     struct kvm_enable_cap *cap)
 {
···
 		r = __set_sregs2(vcpu, u.sregs2);
 		break;
 	}
+	case KVM_HAS_DEVICE_ATTR:
+	case KVM_GET_DEVICE_ATTR:
+	case KVM_SET_DEVICE_ATTR:
+		r = kvm_vcpu_ioctl_device_attr(vcpu, ioctl, argp);
+		break;
 	default:
 		r = -EINVAL;
 	}