6 files changed, 346 insertions, 35 deletions

diff --git a/virt/kvm/coalesced_mmio.c b/virt/kvm/coalesced_mmio.c
new file mode 100644
index 000000000000..5ae620d32fac
--- /dev/null
+++ b/virt/kvm/coalesced_mmio.c
@@ -0,0 +1,156 @@
+/*
+ * KVM coalesced MMIO
+ *
+ * Copyright (c) 2008 Bull S.A.S.
+ *
+ *  Author: Laurent Vivier <Laurent.Vivier@bull.net>
+ *
+ */
+
+#include "iodev.h"
+
+#include <linux/kvm_host.h>
+#include <linux/kvm.h>
+
+#include "coalesced_mmio.h"
+
+static int coalesced_mmio_in_range(struct kvm_io_device *this,
+                                   gpa_t addr, int len, int is_write)
+{
+        struct kvm_coalesced_mmio_dev *dev =
+                                (struct kvm_coalesced_mmio_dev*)this->private;
+        struct kvm_coalesced_mmio_zone *zone;
+        int next;
+        int i;
+
+        if (!is_write)
+                return 0;
+
+        /* kvm->lock is taken by the caller and must be not released before
+         * dev.read/write
+         */
+
+        /* Are we able to batch it ? */
+
+        /* last is the first free entry
+         * check if we don't meet the first used entry
+         * there is always one unused entry in the buffer
+         */
+
+        next = (dev->kvm->coalesced_mmio_ring->last + 1) %
+                                                        KVM_COALESCED_MMIO_MAX;
+        if (next == dev->kvm->coalesced_mmio_ring->first) {
+                /* full */
+                return 0;
+        }
+
+        /* is it in a batchable area ? */
+
+        for (i = 0; i < dev->nb_zones; i++) {
+                zone = &dev->zone[i];
+
+                /* (addr,len) is fully included in
+                 * (zone->addr, zone->size)
+                 */
+
+                if (zone->addr <= addr &&
+                    addr + len <= zone->addr + zone->size)
+                        return 1;
+        }
+        return 0;
+}
+
+static void coalesced_mmio_write(struct kvm_io_device *this,
+                                 gpa_t addr, int len, const void *val)
+{
+        struct kvm_coalesced_mmio_dev *dev =
+                                (struct kvm_coalesced_mmio_dev*)this->private;
+        struct kvm_coalesced_mmio_ring *ring = dev->kvm->coalesced_mmio_ring;
+
+        /* kvm->lock must be taken by caller before call to in_range()*/
+
+        /* copy data in first free entry of the ring */
+
+        ring->coalesced_mmio[ring->last].phys_addr = addr;
+        ring->coalesced_mmio[ring->last].len = len;
+        memcpy(ring->coalesced_mmio[ring->last].data, val, len);
+        smp_wmb();
+        ring->last = (ring->last + 1) % KVM_COALESCED_MMIO_MAX;
+}
+
+static void coalesced_mmio_destructor(struct kvm_io_device *this)
+{
+        kfree(this);
+}
+
+int kvm_coalesced_mmio_init(struct kvm *kvm)
+{
+        struct kvm_coalesced_mmio_dev *dev;
+
+        dev = kzalloc(sizeof(struct kvm_coalesced_mmio_dev), GFP_KERNEL);
+        if (!dev)
+                return -ENOMEM;
+        dev->dev.write  = coalesced_mmio_write;
+        dev->dev.in_range  = coalesced_mmio_in_range;
+        dev->dev.destructor  = coalesced_mmio_destructor;
+        dev->dev.private  = dev;
+        dev->kvm = kvm;
+        kvm->coalesced_mmio_dev = dev;
+        kvm_io_bus_register_dev(&kvm->mmio_bus, &dev->dev);
+
+        return 0;
+}
+
+int kvm_vm_ioctl_register_coalesced_mmio(struct kvm *kvm,
+                                         struct kvm_coalesced_mmio_zone *zone)
+{
+        struct kvm_coalesced_mmio_dev *dev = kvm->coalesced_mmio_dev;
+
+        if (dev == NULL)
+                return -EINVAL;
+
+        mutex_lock(&kvm->lock);
+        if (dev->nb_zones >= KVM_COALESCED_MMIO_ZONE_MAX) {
+                mutex_unlock(&kvm->lock);
+                return -ENOBUFS;
+        }
+
+        dev->zone[dev->nb_zones] = *zone;
+        dev->nb_zones++;
+
+        mutex_unlock(&kvm->lock);
+        return 0;
+}
+
+int kvm_vm_ioctl_unregister_coalesced_mmio(struct kvm *kvm,
+                                           struct kvm_coalesced_mmio_zone *zone)
+{
+        int i;
+        struct kvm_coalesced_mmio_dev *dev = kvm->coalesced_mmio_dev;
+        struct kvm_coalesced_mmio_zone *z;
+
+        if (dev == NULL)
+                return -EINVAL;
+
+        mutex_lock(&kvm->lock);
+
+        i = dev->nb_zones;
+        while(i) {
+                z = &dev->zone[i - 1];
+
+                /* unregister all zones
+                 * included in (zone->addr, zone->size)
+                 */
+
+                if (zone->addr <= z->addr &&
+                    z->addr + z->size <= zone->addr + zone->size) {
+                        dev->nb_zones--;
+                        *z = dev->zone[dev->nb_zones];
+                }
+                i--;
+        }
+
+        mutex_unlock(&kvm->lock);
+
+        return 0;
+}
diff --git a/virt/kvm/coalesced_mmio.h b/virt/kvm/coalesced_mmio.h
new file mode 100644
index 000000000000..5ac0ec628461
--- /dev/null
+++ b/virt/kvm/coalesced_mmio.h
@@ -0,0 +1,23 @@
+/*
+ * KVM coalesced MMIO
+ *
+ * Copyright (c) 2008 Bull S.A.S.
+ *
+ *  Author: Laurent Vivier <Laurent.Vivier@bull.net>
+ *
+ */
+
+#define KVM_COALESCED_MMIO_ZONE_MAX 100
+
+struct kvm_coalesced_mmio_dev {
+        struct kvm_io_device dev;
+        struct kvm *kvm;
+        int nb_zones;
+        struct kvm_coalesced_mmio_zone zone[KVM_COALESCED_MMIO_ZONE_MAX];
+};
+
+int kvm_coalesced_mmio_init(struct kvm *kvm);
+int kvm_vm_ioctl_register_coalesced_mmio(struct kvm *kvm,
+                                       struct kvm_coalesced_mmio_zone *zone);
+int kvm_vm_ioctl_unregister_coalesced_mmio(struct kvm *kvm,
+                                         struct kvm_coalesced_mmio_zone *zone);
diff --git a/virt/kvm/ioapic.c b/virt/kvm/ioapic.c
index 44589088941f..c0d22870ee9c 100644
--- a/virt/kvm/ioapic.c
+++ b/virt/kvm/ioapic.c
@@ -146,6 +146,11 @@ static int ioapic_inj_irq(struct kvm_ioapic *ioapic,
         return kvm_apic_set_irq(vcpu, vector, trig_mode);
 }
 
+static void ioapic_inj_nmi(struct kvm_vcpu *vcpu)
+{
+        kvm_inject_nmi(vcpu);
+}
+
 static u32 ioapic_get_delivery_bitmask(struct kvm_ioapic *ioapic, u8 dest,
                                        u8 dest_mode)
 {
@@ -239,8 +244,19 @@ static int ioapic_deliver(struct kvm_ioapic *ioapic, int irq)
                         }
                 }
                 break;
-
-                /* TODO: NMI */
+        case IOAPIC_NMI:
+                for (vcpu_id = 0; deliver_bitmask != 0; vcpu_id++) {
+                        if (!(deliver_bitmask & (1 << vcpu_id)))
+                                continue;
+                        deliver_bitmask &= ~(1 << vcpu_id);
+                        vcpu = ioapic->kvm->vcpus[vcpu_id];
+                        if (vcpu)
+                                ioapic_inj_nmi(vcpu);
+                        else
+                                ioapic_debug("NMI to vcpu %d failed\n",
+                                                vcpu->vcpu_id);
+                }
+                break;
         default:
                 printk(KERN_WARNING "Unsupported delivery mode %d\n",
                        delivery_mode);
@@ -291,7 +307,8 @@ void kvm_ioapic_update_eoi(struct kvm *kvm, int vector)
                         __kvm_ioapic_update_eoi(ioapic, i);
 }
 
-static int ioapic_in_range(struct kvm_io_device *this, gpa_t addr)
+static int ioapic_in_range(struct kvm_io_device *this, gpa_t addr,
+                           int len, int is_write)
 {
         struct kvm_ioapic *ioapic = (struct kvm_ioapic *)this->private;
 
diff --git a/virt/kvm/iodev.h b/virt/kvm/iodev.h
index c14e642027b2..55e8846ac3a6 100644
--- a/virt/kvm/iodev.h
+++ b/virt/kvm/iodev.h
@@ -27,7 +27,8 @@ struct kvm_io_device {
                       gpa_t addr,
                       int len,
                       const void *val);
-        int (*in_range)(struct kvm_io_device *this, gpa_t addr);
+        int (*in_range)(struct kvm_io_device *this, gpa_t addr, int len,
+                        int is_write);
         void (*destructor)(struct kvm_io_device *this);
 
         void             *private;
@@ -49,9 +50,10 @@ static inline void kvm_iodevice_write(struct kvm_io_device *dev,
         dev->write(dev, addr, len, val);
 }
 
-static inline int kvm_iodevice_inrange(struct kvm_io_device *dev, gpa_t addr)
+static inline int kvm_iodevice_inrange(struct kvm_io_device *dev,
+                                       gpa_t addr, int len, int is_write)
 {
-        return dev->in_range(dev, addr);
+        return dev->in_range(dev, addr, len, is_write);
 }
 
 static inline void kvm_iodevice_destructor(struct kvm_io_device *dev)
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index d4eae6af0738..904d7b7bd780 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -47,6 +47,10 @@
 #include <asm/uaccess.h>
 #include <asm/pgtable.h>
 
+#ifdef KVM_COALESCED_MMIO_PAGE_OFFSET
+#include "coalesced_mmio.h"
+#endif
+
 MODULE_AUTHOR("Qumranet");
 MODULE_LICENSE("GPL");
 
@@ -65,6 +69,8 @@ struct dentry *kvm_debugfs_dir;
 static long kvm_vcpu_ioctl(struct file *file, unsigned int ioctl,
                            unsigned long arg);
 
+bool kvm_rebooting;
+
 static inline int valid_vcpu(int n)
 {
         return likely(n >= 0 && n < KVM_MAX_VCPUS);
@@ -99,10 +105,11 @@ static void ack_flush(void *_completed)
 
 void kvm_flush_remote_tlbs(struct kvm *kvm)
 {
-        int i, cpu;
+        int i, cpu, me;
         cpumask_t cpus;
         struct kvm_vcpu *vcpu;
 
+        me = get_cpu();
         cpus_clear(cpus);
         for (i = 0; i < KVM_MAX_VCPUS; ++i) {
                 vcpu = kvm->vcpus[i];
@@ -111,21 +118,24 @@ void kvm_flush_remote_tlbs(struct kvm *kvm)
                 if (test_and_set_bit(KVM_REQ_TLB_FLUSH, &vcpu->requests))
                         continue;
                 cpu = vcpu->cpu;
-                if (cpu != -1 && cpu != raw_smp_processor_id())
+                if (cpu != -1 && cpu != me)
                         cpu_set(cpu, cpus);
         }
         if (cpus_empty(cpus))
-                return;
+                goto out;
         ++kvm->stat.remote_tlb_flush;
         smp_call_function_mask(cpus, ack_flush, NULL, 1);
+out:
+        put_cpu();
 }
 
 void kvm_reload_remote_mmus(struct kvm *kvm)
 {
-        int i, cpu;
+        int i, cpu, me;
         cpumask_t cpus;
         struct kvm_vcpu *vcpu;
 
+        me = get_cpu();
         cpus_clear(cpus);
         for (i = 0; i < KVM_MAX_VCPUS; ++i) {
                 vcpu = kvm->vcpus[i];
@@ -134,12 +144,14 @@ void kvm_reload_remote_mmus(struct kvm *kvm)
                 if (test_and_set_bit(KVM_REQ_MMU_RELOAD, &vcpu->requests))
                         continue;
                 cpu = vcpu->cpu;
-                if (cpu != -1 && cpu != raw_smp_processor_id())
+                if (cpu != -1 && cpu != me)
                         cpu_set(cpu, cpus);
         }
         if (cpus_empty(cpus))
-                return;
+                goto out;
         smp_call_function_mask(cpus, ack_flush, NULL, 1);
+out:
+        put_cpu();
 }
 
 
@@ -183,10 +195,23 @@ EXPORT_SYMBOL_GPL(kvm_vcpu_uninit);
 static struct kvm *kvm_create_vm(void)
 {
         struct kvm *kvm = kvm_arch_create_vm();
+#ifdef KVM_COALESCED_MMIO_PAGE_OFFSET
+        struct page *page;
+#endif
 
         if (IS_ERR(kvm))
                 goto out;
 
+#ifdef KVM_COALESCED_MMIO_PAGE_OFFSET
+        page = alloc_page(GFP_KERNEL | __GFP_ZERO);
+        if (!page) {
+                kfree(kvm);
+                return ERR_PTR(-ENOMEM);
+        }
+        kvm->coalesced_mmio_ring =
+                        (struct kvm_coalesced_mmio_ring *)page_address(page);
+#endif
+
         kvm->mm = current->mm;
         atomic_inc(&kvm->mm->mm_count);
         spin_lock_init(&kvm->mmu_lock);
@@ -198,6 +223,9 @@ static struct kvm *kvm_create_vm(void)
         spin_lock(&kvm_lock);
         list_add(&kvm->vm_list, &vm_list);
         spin_unlock(&kvm_lock);
+#ifdef KVM_COALESCED_MMIO_PAGE_OFFSET
+        kvm_coalesced_mmio_init(kvm);
+#endif
 out:
         return kvm;
 }
@@ -240,6 +268,10 @@ static void kvm_destroy_vm(struct kvm *kvm)
         spin_unlock(&kvm_lock);
         kvm_io_bus_destroy(&kvm->pio_bus);
         kvm_io_bus_destroy(&kvm->mmio_bus);
+#ifdef KVM_COALESCED_MMIO_PAGE_OFFSET
+        if (kvm->coalesced_mmio_ring != NULL)
+                free_page((unsigned long)kvm->coalesced_mmio_ring);
+#endif
         kvm_arch_destroy_vm(kvm);
         mmdrop(mm);
 }
@@ -333,6 +365,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
         r = -ENOMEM;
 
         /* Allocate if a slot is being created */
+#ifndef CONFIG_S390
         if (npages && !new.rmap) {
                 new.rmap = vmalloc(npages * sizeof(struct page *));
 
@@ -373,10 +406,14 @@ int __kvm_set_memory_region(struct kvm *kvm,
                         goto out_free;
                 memset(new.dirty_bitmap, 0, dirty_bytes);
         }
+#endif /* not defined CONFIG_S390 */
 
         if (mem->slot >= kvm->nmemslots)
                 kvm->nmemslots = mem->slot + 1;
 
+        if (!npages)
+                kvm_arch_flush_shadow(kvm);
+
         *memslot = new;
 
         r = kvm_arch_set_memory_region(kvm, mem, old, user_alloc);
@@ -532,6 +569,7 @@ pfn_t gfn_to_pfn(struct kvm *kvm, gfn_t gfn)
         struct page *page[1];
         unsigned long addr;
         int npages;
+        pfn_t pfn;
 
         might_sleep();
 
@@ -544,19 +582,38 @@ pfn_t gfn_to_pfn(struct kvm *kvm, gfn_t gfn)
         npages = get_user_pages(current, current->mm, addr, 1, 1, 1, page,
                                 NULL);
 
-        if (npages != 1) {
-                get_page(bad_page);
-                return page_to_pfn(bad_page);
-        }
+        if (unlikely(npages != 1)) {
+                struct vm_area_struct *vma;
 
-        return page_to_pfn(page[0]);
+                vma = find_vma(current->mm, addr);
+                if (vma == NULL || addr < vma->vm_start ||
+                    !(vma->vm_flags & VM_PFNMAP)) {
+                        get_page(bad_page);
+                        return page_to_pfn(bad_page);
+                }
+
+                pfn = ((addr - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
+                BUG_ON(pfn_valid(pfn));
+        } else
+                pfn = page_to_pfn(page[0]);
+
+        return pfn;
 }
 
 EXPORT_SYMBOL_GPL(gfn_to_pfn);
 
 struct page *gfn_to_page(struct kvm *kvm, gfn_t gfn)
 {
-        return pfn_to_page(gfn_to_pfn(kvm, gfn));
+        pfn_t pfn;
+
+        pfn = gfn_to_pfn(kvm, gfn);
+        if (pfn_valid(pfn))
+                return pfn_to_page(pfn);
+
+        WARN_ON(!pfn_valid(pfn));
+
+        get_page(bad_page);
+        return bad_page;
 }
 
 EXPORT_SYMBOL_GPL(gfn_to_page);
@@ -569,7 +626,8 @@ EXPORT_SYMBOL_GPL(kvm_release_page_clean);
 
 void kvm_release_pfn_clean(pfn_t pfn)
 {
-        put_page(pfn_to_page(pfn));
+        if (pfn_valid(pfn))
+                put_page(pfn_to_page(pfn));
 }
 EXPORT_SYMBOL_GPL(kvm_release_pfn_clean);
 
@@ -594,21 +652,25 @@ EXPORT_SYMBOL_GPL(kvm_set_page_dirty);
 
 void kvm_set_pfn_dirty(pfn_t pfn)
 {
-        struct page *page = pfn_to_page(pfn);
-        if (!PageReserved(page))
-                SetPageDirty(page);
+        if (pfn_valid(pfn)) {
+                struct page *page = pfn_to_page(pfn);
+                if (!PageReserved(page))
+                        SetPageDirty(page);
+        }
 }
 EXPORT_SYMBOL_GPL(kvm_set_pfn_dirty);
 
 void kvm_set_pfn_accessed(pfn_t pfn)
 {
-        mark_page_accessed(pfn_to_page(pfn));
+        if (pfn_valid(pfn))
+                mark_page_accessed(pfn_to_page(pfn));
 }
 EXPORT_SYMBOL_GPL(kvm_set_pfn_accessed);
 
 void kvm_get_pfn(pfn_t pfn)
 {
-        get_page(pfn_to_page(pfn));
+        if (pfn_valid(pfn))
+                get_page(pfn_to_page(pfn));
 }
 EXPORT_SYMBOL_GPL(kvm_get_pfn);
 
@@ -799,6 +861,10 @@ static int kvm_vcpu_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
         else if (vmf->pgoff == KVM_PIO_PAGE_OFFSET)
                 page = virt_to_page(vcpu->arch.pio_data);
 #endif
+#ifdef KVM_COALESCED_MMIO_PAGE_OFFSET
+        else if (vmf->pgoff == KVM_COALESCED_MMIO_PAGE_OFFSET)
+                page = virt_to_page(vcpu->kvm->coalesced_mmio_ring);
+#endif
         else
                 return VM_FAULT_SIGBUS;
         get_page(page);
@@ -1121,6 +1187,32 @@ static long kvm_vm_ioctl(struct file *filp,
                         goto out;
                 break;
         }
+#ifdef KVM_COALESCED_MMIO_PAGE_OFFSET
+        case KVM_REGISTER_COALESCED_MMIO: {
+                struct kvm_coalesced_mmio_zone zone;
+                r = -EFAULT;
+                if (copy_from_user(&zone, argp, sizeof zone))
+                        goto out;
+                r = -ENXIO;
+                r = kvm_vm_ioctl_register_coalesced_mmio(kvm, &zone);
+                if (r)
+                        goto out;
+                r = 0;
+                break;
+        }
+        case KVM_UNREGISTER_COALESCED_MMIO: {
+                struct kvm_coalesced_mmio_zone zone;
+                r = -EFAULT;
+                if (copy_from_user(&zone, argp, sizeof zone))
+                        goto out;
+                r = -ENXIO;
+                r = kvm_vm_ioctl_unregister_coalesced_mmio(kvm, &zone);
+                if (r)
+                        goto out;
+                r = 0;
+                break;
+        }
+#endif
         default:
                 r = kvm_arch_vm_ioctl(filp, ioctl, arg);
         }
@@ -1179,7 +1271,6 @@ static int kvm_dev_ioctl_create_vm(void)
 static long kvm_dev_ioctl(struct file *filp,
                           unsigned int ioctl, unsigned long arg)
 {
-        void __user *argp = (void __user *)arg;
         long r = -EINVAL;
 
         switch (ioctl) {
@@ -1196,7 +1287,7 @@ static long kvm_dev_ioctl(struct file *filp,
                 r = kvm_dev_ioctl_create_vm();
                 break;
         case KVM_CHECK_EXTENSION:
-                r = kvm_dev_ioctl_check_extension((long)argp);
+                r = kvm_dev_ioctl_check_extension(arg);
                 break;
         case KVM_GET_VCPU_MMAP_SIZE:
                 r = -EINVAL;
@@ -1206,6 +1297,9 @@ static long kvm_dev_ioctl(struct file *filp,
 #ifdef CONFIG_X86
                 r += PAGE_SIZE;    /* pio data page */
 #endif
+#ifdef KVM_COALESCED_MMIO_PAGE_OFFSET
+                r += PAGE_SIZE;    /* coalesced mmio ring page */
+#endif
                 break;
         case KVM_TRACE_ENABLE:
         case KVM_TRACE_PAUSE:
@@ -1247,7 +1341,6 @@ static void hardware_disable(void *junk)
         if (!cpu_isset(cpu, cpus_hardware_enabled))
                 return;
         cpu_clear(cpu, cpus_hardware_enabled);
-        decache_vcpus_on_cpu(cpu);
         kvm_arch_hardware_disable(NULL);
 }
 
@@ -1277,6 +1370,18 @@ static int kvm_cpu_hotplug(struct notifier_block *notifier, unsigned long val,
         return NOTIFY_OK;
 }
 
+
+asmlinkage void kvm_handle_fault_on_reboot(void)
+{
+        if (kvm_rebooting)
+                /* spin while reset goes on */
+                while (true)
+                        ;
+        /* Fault while not rebooting.  We want the trace. */
+        BUG();
+}
+EXPORT_SYMBOL_GPL(kvm_handle_fault_on_reboot);
+
 static int kvm_reboot(struct notifier_block *notifier, unsigned long val,
                       void *v)
 {
@@ -1286,6 +1391,7 @@ static int kvm_reboot(struct notifier_block *notifier, unsigned long val,
                  * in vmx root mode.
                  */
                 printk(KERN_INFO "kvm: exiting hardware virtualization\n");
+                kvm_rebooting = true;
                 on_each_cpu(hardware_disable, NULL, 1);
         }
         return NOTIFY_OK;
@@ -1312,14 +1418,15 @@ void kvm_io_bus_destroy(struct kvm_io_bus *bus)
         }
 }
 
-struct kvm_io_device *kvm_io_bus_find_dev(struct kvm_io_bus *bus, gpa_t addr)
+struct kvm_io_device *kvm_io_bus_find_dev(struct kvm_io_bus *bus,
+                                          gpa_t addr, int len, int is_write)
 {
         int i;
 
         for (i = 0; i < bus->dev_count; i++) {
                 struct kvm_io_device *pos = bus->devs[i];
 
-                if (pos->in_range(pos, addr))
+                if (pos->in_range(pos, addr, len, is_write))
                         return pos;
         }
 
diff --git a/virt/kvm/kvm_trace.c b/virt/kvm/kvm_trace.c
index 0e495470788d..58141f31ea8f 100644
--- a/virt/kvm/kvm_trace.c
+++ b/virt/kvm/kvm_trace.c
@@ -72,11 +72,7 @@ static void kvm_add_trace(void *probe_private, void *call_data,
         rec.cycle_in    = p->cycle_in;
 
         if (rec.cycle_in) {
-                u64 cycle = 0;
-
-                cycle = get_cycles();
-                rec.u.cycle.cycle_lo = (u32)cycle;
-                rec.u.cycle.cycle_hi = (u32)(cycle >> 32);
+                rec.u.cycle.cycle_u64 = get_cycles();
 
                 for (i = 0; i < rec.extra_u32; i++)
                         rec.u.cycle.extra_u32[i] = va_arg(*args, u32);
@@ -114,8 +110,18 @@ static int kvm_subbuf_start_callback(struct rchan_buf *buf, void *subbuf,
 {
         struct kvm_trace *kt;
 
-        if (!relay_buf_full(buf))
+        if (!relay_buf_full(buf)) {
+                if (!prev_subbuf) {
+                        /*
+                         * executed only once when the channel is opened
+                         * save metadata as first record
+                         */
+                        subbuf_start_reserve(buf, sizeof(u32));
+                        *(u32 *)subbuf = 0x12345678;
+                }
+
                 return 1;
+        }
 
         kt = buf->chan->private_data;
         atomic_inc(&kt->lost_records);