Re: [PATCH v2] target/i386: always create kvmclock device

2020-09-24 Thread Dr. David Alan Gilbert 
* Vitaly Kuznetsov (vkuzn...@redhat.com) wrote:
> QEMU's kvmclock device is only created when KVM PV feature bits for
> kvmclock (KVM_FEATURE_CLOCKSOURCE/KVM_FEATURE_CLOCKSOURCE2) are
> exposed to the guest. With 'kvm=off' cpu flag the device is not
> created and we don't call KVM_GET_CLOCK/KVM_SET_CLOCK upon migration.
> It was reported that without these call at least Hyper-V TSC page
> clocksouce (which can be enabled independently) gets broken after
> migration.
> 
> Switch to creating kvmclock QEMU device unconditionally, it seems
> to always make sense to call KVM_GET_CLOCK/KVM_SET_CLOCK on migration.
> Use KVM_CAP_ADJUST_CLOCK check instead of CPUID feature bits.
> 
> Reported-by: Antoine Damhet 
> Suggested-by: Paolo Bonzini 
> Signed-off-by: Vitaly Kuznetsov 
> ---
>  hw/i386/kvm/clock.c| 7 +--
>  hw/i386/microvm.c  | 2 +-
>  hw/i386/pc.c   | 1 +
>  hw/i386/pc_piix.c  | 7 +--
>  hw/i386/pc_q35.c   | 5 -
>  include/hw/i386/pc.h   | 3 +++
>  include/hw/kvm/clock.h | 4 ++--
>  target/i386/kvm.c  | 5 +
>  target/i386/kvm_i386.h | 1 +
>  9 files changed, 27 insertions(+), 8 deletions(-)
> 
> diff --git a/hw/i386/kvm/clock.c b/hw/i386/kvm/clock.c
> index 64283358f91d..30cf53393ee1 100644
> --- a/hw/i386/kvm/clock.c
> +++ b/hw/i386/kvm/clock.c
> @@ -328,11 +328,14 @@ static const TypeInfo kvmclock_info = {
>  };
>  
>  /* Note: Must be called after VCPU initialization. */
> -void kvmclock_create(void)
> +void kvmclock_create(bool create_always)
>  {
>  X86CPU *cpu = X86_CPU(first_cpu);
>  
> -if (kvm_enabled() &&
> +if (!kvm_enabled() || !kvm_has_adjust_clock())
> +return;
> +
> +if (create_always ||
>  cpu->env.features[FEAT_KVM] & ((1ULL << KVM_FEATURE_CLOCKSOURCE) |
> (1ULL << KVM_FEATURE_CLOCKSOURCE2))) {
>  sysbus_create_simple(TYPE_KVM_CLOCK, -1, NULL);
> diff --git a/hw/i386/microvm.c b/hw/i386/microvm.c
> index 81d0888930d1..fd0b84109154 100644
> --- a/hw/i386/microvm.c
> +++ b/hw/i386/microvm.c
> @@ -119,7 +119,7 @@ static void microvm_devices_init(MicrovmMachineState *mms)
>  
>  ioapic_init_gsi(gsi_state, "machine");
>  
> -kvmclock_create();
> +kvmclock_create(true);
>  
>  for (i = 0; i < VIRTIO_NUM_TRANSPORTS; i++) {
>  sysbus_create_simple("virtio-mmio",
> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> index d11daacc23cf..0e036ef9c15f 100644
> --- a/hw/i386/pc.c
> +++ b/hw/i386/pc.c
> @@ -1956,6 +1956,7 @@ static void pc_machine_class_init(ObjectClass *oc, void 
> *data)
>  pcmc->acpi_data_size = 0x2 + 0x8000;
>  pcmc->linuxboot_dma_enabled = true;
>  pcmc->pvh_enabled = true;
> +pcmc->kvmclock_create_always = true;
>  assert(!mc->get_hotplug_handler);
>  mc->get_hotplug_handler = pc_get_hotplug_handler;
>  mc->hotplug_allowed = pc_hotplug_allowed;
> diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
> index 32b1453e6a82..1a68338c737b 100644
> --- a/hw/i386/pc_piix.c
> +++ b/hw/i386/pc_piix.c
> @@ -158,8 +158,8 @@ static void pc_init1(MachineState *machine,
>  
>  x86_cpus_init(x86ms, pcmc->default_cpu_version);
>  
> -if (kvm_enabled() && pcmc->kvmclock_enabled) {
> -kvmclock_create();
> +if (pcmc->kvmclock_enabled) {
> +kvmclock_create(pcmc->kvmclock_create_always);
>  }
>  
>  if (pcmc->pci_enabled) {
> @@ -440,11 +440,14 @@ DEFINE_I440FX_MACHINE(v5_2, "pc-i440fx-5.2", NULL,
>  
>  static void pc_i440fx_5_1_machine_options(MachineClass *m)
>  {
> +PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
> +
>  pc_i440fx_5_2_machine_options(m);
>  m->alias = NULL;
>  m->is_default = false;
>  compat_props_add(m->compat_props, hw_compat_5_1, hw_compat_5_1_len);
>  compat_props_add(m->compat_props, pc_compat_5_1, pc_compat_5_1_len);
> +pcmc->kvmclock_create_always = false;

Thanks; yes that should keep backwards migration happy.

Dave

>  }
>  
>  DEFINE_I440FX_MACHINE(v5_1, "pc-i440fx-5.1", NULL,
> diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
> index 0cb9c18cd44d..0dd59bd765b1 100644
> --- a/hw/i386/pc_q35.c
> +++ b/hw/i386/pc_q35.c
> @@ -185,7 +185,7 @@ static void pc_q35_init(MachineState *machine)
>  
>  x86_cpus_init(x86ms, pcmc->default_cpu_version);
>  
> -kvmclock_create();
> +kvmclock_create(pcmc->kvmclock_create_always);
>  
>  /* pci enabled */
>  if (pcmc->pci_enabled) {
> @@ -366,10 +366,13 @@ DEFINE_Q35_MACHINE(v5_2, "pc-q35-5.2", NULL,
>  
>  static void pc_q35_5_1_machine_options(MachineClass *m)
>  {
> +PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
> +
>  pc_q35_5_2_machine_options(m);
>  m->alias = NULL;
>  compat_props_add(m->compat_props, hw_compat_5_1, hw_compat_5_1_len);
>  compat_props_add(m->compat_props, pc_compat_5_1, pc_compat_5_1_len);
> +pcmc->kvmclock_create_always = false;
>  }
>  
>  DEFINE_Q35_MACHINE(v5_1, "pc-q35-5.1", NULL,
> diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
>

Re: [PATCH v2] target/i386: always create kvmclock device

2020-09-23 Thread Antoine Damhet 
Hi,

The patch doesn't apply cleanly to master but it works (patched ->
patched working, patched -> unpatched: old behavior, unpatched ->
patched: old behavior)

Thanks to everyone for the swift responses :)

On Tue, Sep 22, 2020 at 05:19:34PM +0200, Vitaly Kuznetsov wrote:
> QEMU's kvmclock device is only created when KVM PV feature bits for
> kvmclock (KVM_FEATURE_CLOCKSOURCE/KVM_FEATURE_CLOCKSOURCE2) are
> exposed to the guest. With 'kvm=off' cpu flag the device is not
> created and we don't call KVM_GET_CLOCK/KVM_SET_CLOCK upon migration.
> It was reported that without these call at least Hyper-V TSC page
> clocksouce (which can be enabled independently) gets broken after
> migration.
> 
> Switch to creating kvmclock QEMU device unconditionally, it seems
> to always make sense to call KVM_GET_CLOCK/KVM_SET_CLOCK on migration.
> Use KVM_CAP_ADJUST_CLOCK check instead of CPUID feature bits.
> 
> Reported-by: Antoine Damhet 
> Suggested-by: Paolo Bonzini 
> Signed-off-by: Vitaly Kuznetsov 

Tested-by: Antoine Damhet 

> ---
>  hw/i386/kvm/clock.c| 7 +--
>  hw/i386/microvm.c  | 2 +-
>  hw/i386/pc.c   | 1 +
>  hw/i386/pc_piix.c  | 7 +--
>  hw/i386/pc_q35.c   | 5 -
>  include/hw/i386/pc.h   | 3 +++
>  include/hw/kvm/clock.h | 4 ++--
>  target/i386/kvm.c  | 5 +
>  target/i386/kvm_i386.h | 1 +
>  9 files changed, 27 insertions(+), 8 deletions(-)
> 
> diff --git a/hw/i386/kvm/clock.c b/hw/i386/kvm/clock.c
> index 64283358f91d..30cf53393ee1 100644
> --- a/hw/i386/kvm/clock.c
> +++ b/hw/i386/kvm/clock.c
> @@ -328,11 +328,14 @@ static const TypeInfo kvmclock_info = {
>  };
>  
>  /* Note: Must be called after VCPU initialization. */
> -void kvmclock_create(void)
> +void kvmclock_create(bool create_always)
>  {
>  X86CPU *cpu = X86_CPU(first_cpu);
>  
> -if (kvm_enabled() &&
> +if (!kvm_enabled() || !kvm_has_adjust_clock())
> +return;
> +
> +if (create_always ||
>  cpu->env.features[FEAT_KVM] & ((1ULL << KVM_FEATURE_CLOCKSOURCE) |
> (1ULL << KVM_FEATURE_CLOCKSOURCE2))) {
>  sysbus_create_simple(TYPE_KVM_CLOCK, -1, NULL);
> diff --git a/hw/i386/microvm.c b/hw/i386/microvm.c
> index 81d0888930d1..fd0b84109154 100644
> --- a/hw/i386/microvm.c
> +++ b/hw/i386/microvm.c
> @@ -119,7 +119,7 @@ static void microvm_devices_init(MicrovmMachineState *mms)
>  
>  ioapic_init_gsi(gsi_state, "machine");
>  
> -kvmclock_create();
> +kvmclock_create(true);
>  
>  for (i = 0; i < VIRTIO_NUM_TRANSPORTS; i++) {
>  sysbus_create_simple("virtio-mmio",
> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> index d11daacc23cf..0e036ef9c15f 100644
> --- a/hw/i386/pc.c
> +++ b/hw/i386/pc.c
> @@ -1956,6 +1956,7 @@ static void pc_machine_class_init(ObjectClass *oc, void 
> *data)
>  pcmc->acpi_data_size = 0x2 + 0x8000;
>  pcmc->linuxboot_dma_enabled = true;
>  pcmc->pvh_enabled = true;
> +pcmc->kvmclock_create_always = true;
>  assert(!mc->get_hotplug_handler);
>  mc->get_hotplug_handler = pc_get_hotplug_handler;
>  mc->hotplug_allowed = pc_hotplug_allowed;
> diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
> index 32b1453e6a82..1a68338c737b 100644
> --- a/hw/i386/pc_piix.c
> +++ b/hw/i386/pc_piix.c
> @@ -158,8 +158,8 @@ static void pc_init1(MachineState *machine,
>  
>  x86_cpus_init(x86ms, pcmc->default_cpu_version);
>  
> -if (kvm_enabled() && pcmc->kvmclock_enabled) {
> -kvmclock_create();
> +if (pcmc->kvmclock_enabled) {
> +kvmclock_create(pcmc->kvmclock_create_always);
>  }
>  
>  if (pcmc->pci_enabled) {
> @@ -440,11 +440,14 @@ DEFINE_I440FX_MACHINE(v5_2, "pc-i440fx-5.2", NULL,
>  
>  static void pc_i440fx_5_1_machine_options(MachineClass *m)
>  {
> +PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
> +
>  pc_i440fx_5_2_machine_options(m);
>  m->alias = NULL;
>  m->is_default = false;
>  compat_props_add(m->compat_props, hw_compat_5_1, hw_compat_5_1_len);
>  compat_props_add(m->compat_props, pc_compat_5_1, pc_compat_5_1_len);
> +pcmc->kvmclock_create_always = false;
>  }
>  
>  DEFINE_I440FX_MACHINE(v5_1, "pc-i440fx-5.1", NULL,
> diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
> index 0cb9c18cd44d..0dd59bd765b1 100644
> --- a/hw/i386/pc_q35.c
> +++ b/hw/i386/pc_q35.c
> @@ -185,7 +185,7 @@ static void pc_q35_init(MachineState *machine)
>  
>  x86_cpus_init(x86ms, pcmc->default_cpu_version);
>  
> -kvmclock_create();
> +kvmclock_create(pcmc->kvmclock_create_always);
>  
>  /* pci enabled */
>  if (pcmc->pci_enabled) {
> @@ -366,10 +366,13 @@ DEFINE_Q35_MACHINE(v5_2, "pc-q35-5.2", NULL,
>  
>  static void pc_q35_5_1_machine_options(MachineClass *m)
>  {
> +PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
> +
>  pc_q35_5_2_machine_options(m);
>  m->alias = NULL;
>  compat_props_add(m->compat_props, hw_compat_5_1, hw_compat_5_1_len);
>  compat_props_add(m->compat_props,

Re: [PATCH v2] target/i386: always create kvmclock device

2020-09-22 Thread Paolo Bonzini 
On 22/09/20 17:19, Vitaly Kuznetsov wrote:
> QEMU's kvmclock device is only created when KVM PV feature bits for
> kvmclock (KVM_FEATURE_CLOCKSOURCE/KVM_FEATURE_CLOCKSOURCE2) are
> exposed to the guest. With 'kvm=off' cpu flag the device is not
> created and we don't call KVM_GET_CLOCK/KVM_SET_CLOCK upon migration.
> It was reported that without these call at least Hyper-V TSC page
> clocksouce (which can be enabled independently) gets broken after
> migration.
> 
> Switch to creating kvmclock QEMU device unconditionally, it seems
> to always make sense to call KVM_GET_CLOCK/KVM_SET_CLOCK on migration.
> Use KVM_CAP_ADJUST_CLOCK check instead of CPUID feature bits.
> 
> Reported-by: Antoine Damhet 
> Suggested-by: Paolo Bonzini 
> Signed-off-by: Vitaly Kuznetsov 
> ---
>  hw/i386/kvm/clock.c| 7 +--
>  hw/i386/microvm.c  | 2 +-
>  hw/i386/pc.c   | 1 +
>  hw/i386/pc_piix.c  | 7 +--
>  hw/i386/pc_q35.c   | 5 -
>  include/hw/i386/pc.h   | 3 +++
>  include/hw/kvm/clock.h | 4 ++--
>  target/i386/kvm.c  | 5 +
>  target/i386/kvm_i386.h | 1 +
>  9 files changed, 27 insertions(+), 8 deletions(-)
> 
> diff --git a/hw/i386/kvm/clock.c b/hw/i386/kvm/clock.c
> index 64283358f91d..30cf53393ee1 100644
> --- a/hw/i386/kvm/clock.c
> +++ b/hw/i386/kvm/clock.c
> @@ -328,11 +328,14 @@ static const TypeInfo kvmclock_info = {
>  };
>  
>  /* Note: Must be called after VCPU initialization. */
> -void kvmclock_create(void)
> +void kvmclock_create(bool create_always)
>  {
>  X86CPU *cpu = X86_CPU(first_cpu);
>  
> -if (kvm_enabled() &&
> +if (!kvm_enabled() || !kvm_has_adjust_clock())
> +return;
> +
> +if (create_always ||
>  cpu->env.features[FEAT_KVM] & ((1ULL << KVM_FEATURE_CLOCKSOURCE) |
> (1ULL << KVM_FEATURE_CLOCKSOURCE2))) {
>  sysbus_create_simple(TYPE_KVM_CLOCK, -1, NULL);
> diff --git a/hw/i386/microvm.c b/hw/i386/microvm.c
> index 81d0888930d1..fd0b84109154 100644
> --- a/hw/i386/microvm.c
> +++ b/hw/i386/microvm.c
> @@ -119,7 +119,7 @@ static void microvm_devices_init(MicrovmMachineState *mms)
>  
>  ioapic_init_gsi(gsi_state, "machine");
>  
> -kvmclock_create();
> +kvmclock_create(true);
>  
>  for (i = 0; i < VIRTIO_NUM_TRANSPORTS; i++) {
>  sysbus_create_simple("virtio-mmio",
> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> index d11daacc23cf..0e036ef9c15f 100644
> --- a/hw/i386/pc.c
> +++ b/hw/i386/pc.c
> @@ -1956,6 +1956,7 @@ static void pc_machine_class_init(ObjectClass *oc, void 
> *data)
>  pcmc->acpi_data_size = 0x2 + 0x8000;
>  pcmc->linuxboot_dma_enabled = true;
>  pcmc->pvh_enabled = true;
> +pcmc->kvmclock_create_always = true;
>  assert(!mc->get_hotplug_handler);
>  mc->get_hotplug_handler = pc_get_hotplug_handler;
>  mc->hotplug_allowed = pc_hotplug_allowed;
> diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
> index 32b1453e6a82..1a68338c737b 100644
> --- a/hw/i386/pc_piix.c
> +++ b/hw/i386/pc_piix.c
> @@ -158,8 +158,8 @@ static void pc_init1(MachineState *machine,
>  
>  x86_cpus_init(x86ms, pcmc->default_cpu_version);
>  
> -if (kvm_enabled() && pcmc->kvmclock_enabled) {
> -kvmclock_create();
> +if (pcmc->kvmclock_enabled) {
> +kvmclock_create(pcmc->kvmclock_create_always);
>  }
>  
>  if (pcmc->pci_enabled) {
> @@ -440,11 +440,14 @@ DEFINE_I440FX_MACHINE(v5_2, "pc-i440fx-5.2", NULL,
>  
>  static void pc_i440fx_5_1_machine_options(MachineClass *m)
>  {
> +PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
> +
>  pc_i440fx_5_2_machine_options(m);
>  m->alias = NULL;
>  m->is_default = false;
>  compat_props_add(m->compat_props, hw_compat_5_1, hw_compat_5_1_len);
>  compat_props_add(m->compat_props, pc_compat_5_1, pc_compat_5_1_len);
> +pcmc->kvmclock_create_always = false;
>  }
>  
>  DEFINE_I440FX_MACHINE(v5_1, "pc-i440fx-5.1", NULL,
> diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
> index 0cb9c18cd44d..0dd59bd765b1 100644
> --- a/hw/i386/pc_q35.c
> +++ b/hw/i386/pc_q35.c
> @@ -185,7 +185,7 @@ static void pc_q35_init(MachineState *machine)
>  
>  x86_cpus_init(x86ms, pcmc->default_cpu_version);
>  
> -kvmclock_create();
> +kvmclock_create(pcmc->kvmclock_create_always);
>  
>  /* pci enabled */
>  if (pcmc->pci_enabled) {
> @@ -366,10 +366,13 @@ DEFINE_Q35_MACHINE(v5_2, "pc-q35-5.2", NULL,
>  
>  static void pc_q35_5_1_machine_options(MachineClass *m)
>  {
> +PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
> +
>  pc_q35_5_2_machine_options(m);
>  m->alias = NULL;
>  compat_props_add(m->compat_props, hw_compat_5_1, hw_compat_5_1_len);
>  compat_props_add(m->compat_props, pc_compat_5_1, pc_compat_5_1_len);
> +pcmc->kvmclock_create_always = false;
>  }
>  
>  DEFINE_Q35_MACHINE(v5_1, "pc-q35-5.1", NULL,
> diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
> index fe52e165b27c..6b22671434b9 100644
> --- a/include/hw/i386/pc.h
>

[PATCH v2] target/i386: always create kvmclock device

2020-09-22 Thread Vitaly Kuznetsov 
QEMU's kvmclock device is only created when KVM PV feature bits for
kvmclock (KVM_FEATURE_CLOCKSOURCE/KVM_FEATURE_CLOCKSOURCE2) are
exposed to the guest. With 'kvm=off' cpu flag the device is not
created and we don't call KVM_GET_CLOCK/KVM_SET_CLOCK upon migration.
It was reported that without these call at least Hyper-V TSC page
clocksouce (which can be enabled independently) gets broken after
migration.

Switch to creating kvmclock QEMU device unconditionally, it seems
to always make sense to call KVM_GET_CLOCK/KVM_SET_CLOCK on migration.
Use KVM_CAP_ADJUST_CLOCK check instead of CPUID feature bits.

Reported-by: Antoine Damhet 
Suggested-by: Paolo Bonzini 
Signed-off-by: Vitaly Kuznetsov 
---
 hw/i386/kvm/clock.c| 7 +--
 hw/i386/microvm.c  | 2 +-
 hw/i386/pc.c   | 1 +
 hw/i386/pc_piix.c  | 7 +--
 hw/i386/pc_q35.c   | 5 -
 include/hw/i386/pc.h   | 3 +++
 include/hw/kvm/clock.h | 4 ++--
 target/i386/kvm.c  | 5 +
 target/i386/kvm_i386.h | 1 +
 9 files changed, 27 insertions(+), 8 deletions(-)

diff --git a/hw/i386/kvm/clock.c b/hw/i386/kvm/clock.c
index 64283358f91d..30cf53393ee1 100644
--- a/hw/i386/kvm/clock.c
+++ b/hw/i386/kvm/clock.c
@@ -328,11 +328,14 @@ static const TypeInfo kvmclock_info = {
 };
 
 /* Note: Must be called after VCPU initialization. */
-void kvmclock_create(void)
+void kvmclock_create(bool create_always)
 {
 X86CPU *cpu = X86_CPU(first_cpu);
 
-if (kvm_enabled() &&
+if (!kvm_enabled() || !kvm_has_adjust_clock())
+return;
+
+if (create_always ||
 cpu->env.features[FEAT_KVM] & ((1ULL << KVM_FEATURE_CLOCKSOURCE) |
(1ULL << KVM_FEATURE_CLOCKSOURCE2))) {
 sysbus_create_simple(TYPE_KVM_CLOCK, -1, NULL);
diff --git a/hw/i386/microvm.c b/hw/i386/microvm.c
index 81d0888930d1..fd0b84109154 100644
--- a/hw/i386/microvm.c
+++ b/hw/i386/microvm.c
@@ -119,7 +119,7 @@ static void microvm_devices_init(MicrovmMachineState *mms)
 
 ioapic_init_gsi(gsi_state, "machine");
 
-kvmclock_create();
+kvmclock_create(true);
 
 for (i = 0; i < VIRTIO_NUM_TRANSPORTS; i++) {
 sysbus_create_simple("virtio-mmio",
diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index d11daacc23cf..0e036ef9c15f 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -1956,6 +1956,7 @@ static void pc_machine_class_init(ObjectClass *oc, void 
*data)
 pcmc->acpi_data_size = 0x2 + 0x8000;
 pcmc->linuxboot_dma_enabled = true;
 pcmc->pvh_enabled = true;
+pcmc->kvmclock_create_always = true;
 assert(!mc->get_hotplug_handler);
 mc->get_hotplug_handler = pc_get_hotplug_handler;
 mc->hotplug_allowed = pc_hotplug_allowed;
diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index 32b1453e6a82..1a68338c737b 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -158,8 +158,8 @@ static void pc_init1(MachineState *machine,
 
 x86_cpus_init(x86ms, pcmc->default_cpu_version);
 
-if (kvm_enabled() && pcmc->kvmclock_enabled) {
-kvmclock_create();
+if (pcmc->kvmclock_enabled) {
+kvmclock_create(pcmc->kvmclock_create_always);
 }
 
 if (pcmc->pci_enabled) {
@@ -440,11 +440,14 @@ DEFINE_I440FX_MACHINE(v5_2, "pc-i440fx-5.2", NULL,
 
 static void pc_i440fx_5_1_machine_options(MachineClass *m)
 {
+PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
+
 pc_i440fx_5_2_machine_options(m);
 m->alias = NULL;
 m->is_default = false;
 compat_props_add(m->compat_props, hw_compat_5_1, hw_compat_5_1_len);
 compat_props_add(m->compat_props, pc_compat_5_1, pc_compat_5_1_len);
+pcmc->kvmclock_create_always = false;
 }
 
 DEFINE_I440FX_MACHINE(v5_1, "pc-i440fx-5.1", NULL,
diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
index 0cb9c18cd44d..0dd59bd765b1 100644
--- a/hw/i386/pc_q35.c
+++ b/hw/i386/pc_q35.c
@@ -185,7 +185,7 @@ static void pc_q35_init(MachineState *machine)
 
 x86_cpus_init(x86ms, pcmc->default_cpu_version);
 
-kvmclock_create();
+kvmclock_create(pcmc->kvmclock_create_always);
 
 /* pci enabled */
 if (pcmc->pci_enabled) {
@@ -366,10 +366,13 @@ DEFINE_Q35_MACHINE(v5_2, "pc-q35-5.2", NULL,
 
 static void pc_q35_5_1_machine_options(MachineClass *m)
 {
+PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
+
 pc_q35_5_2_machine_options(m);
 m->alias = NULL;
 compat_props_add(m->compat_props, hw_compat_5_1, hw_compat_5_1_len);
 compat_props_add(m->compat_props, pc_compat_5_1, pc_compat_5_1_len);
+pcmc->kvmclock_create_always = false;
 }
 
 DEFINE_Q35_MACHINE(v5_1, "pc-q35-5.1", NULL,
diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
index fe52e165b27c..6b22671434b9 100644
--- a/include/hw/i386/pc.h
+++ b/include/hw/i386/pc.h
@@ -118,6 +118,9 @@ typedef struct PCMachineClass {
 
 /* use PVH to load kernels that support this feature */
 bool pvh_enabled;
+
+/* create kvmclock device even when KVM PV features are not exposed */
+bool kvmclock_create_always;
 } PCMachineClass;
 
 #define