KVM: x86: pvclock: Handle first-time write to pvclock-page contains random junk

Commit Message

Liran Alon Nov. 5, 2017, 2:11 p.m. UTC

When guest passes KVM it's pvclock-page GPA via WRMSR to
MSR_KVM_SYSTEM_TIME / MSR_KVM_SYSTEM_TIME_NEW, KVM don't initialize
pvclock-page to some start-values. It just requests a clock-update which
will happen before entering to guest.

The clock-update logic will call kvm_setup_pvclock_page() to update the
pvclock-page with info. However, kvm_setup_pvclock_page() *wrongly*
assumes that the version-field is initialized to an even number. This is
wrong because at first-time write, field could be any-value.

Fix simply makes sure that if first-time version-field is odd, increment
it once more to make it even and only then start standard logic.
This follows same logic as done in other pvclock shared-pages (See
kvm_write_wall_clock() and record_steal_time()).

Signed-off-by: Liran Alon <[e-mail]>
Reviewed-by: Nikita Leshenko <[e-mail]>
Reviewed-by: Konrad Rzeszutek Wilk <[e-mail]>
Signed-off-by: Konrad Rzeszutek Wilk <[e-mail]>
---
 arch/x86/kvm/x86.c | 3 +++
 1 file changed, 3 insertions(+)

Comments

Paolo Bonzini Nov. 6, 2017, 9:13 a.m. UTC | #1

On 05/11/2017 15:11, Liran Alon wrote:
> When guest passes KVM it's pvclock-page GPA via WRMSR to
> MSR_KVM_SYSTEM_TIME / MSR_KVM_SYSTEM_TIME_NEW, KVM don't initialize
> pvclock-page to some start-values. It just requests a clock-update which
> will happen before entering to guest.
> 
> The clock-update logic will call kvm_setup_pvclock_page() to update the
> pvclock-page with info. However, kvm_setup_pvclock_page() *wrongly*
> assumes that the version-field is initialized to an even number. This is
> wrong because at first-time write, field could be any-value.
> 
> Fix simply makes sure that if first-time version-field is odd, increment
> it once more to make it even and only then start standard logic.
> This follows same logic as done in other pvclock shared-pages (See
> kvm_write_wall_clock() and record_steal_time()).
> 
> Signed-off-by: Liran Alon <[e-mail]>
> Reviewed-by: Nikita Leshenko <[e-mail]>
> Reviewed-by: Konrad Rzeszutek Wilk <[e-mail]>
> Signed-off-by: Konrad Rzeszutek Wilk <[e-mail]>
> ---
>  arch/x86/kvm/x86.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 03869eb7fcd6..181106080e41 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1830,6 +1830,9 @@ static void kvm_setup_pvclock_page(struct kvm_vcpu *v)
>  	 */
>  	BUILD_BUG_ON(offsetof(struct pvclock_vcpu_time_info, version) != 0);
>  
> +	if (guest_hv_clock.version & 1)
> +		++guest_hv_clock.version;  /* first time write, random junk */
> +
>  	vcpu->hv_clock.version = guest_hv_clock.version + 1;
>  	kvm_write_guest_cached(v->kvm, &vcpu->pv_time,
>  				&vcpu->hv_clock,
> 

Reviewed-by: Paolo Bonzini <[e-mail]>
Cc: [e-mail]

Radim Krčmář Nov. 10, 2017, 9:36 p.m. UTC | #2

2017-11-05 16:11+0200, Liran Alon:
> When guest passes KVM it's pvclock-page GPA via WRMSR to
> MSR_KVM_SYSTEM_TIME / MSR_KVM_SYSTEM_TIME_NEW, KVM don't initialize
> pvclock-page to some start-values. It just requests a clock-update which
> will happen before entering to guest.
> 
> The clock-update logic will call kvm_setup_pvclock_page() to update the
> pvclock-page with info. However, kvm_setup_pvclock_page() *wrongly*
> assumes that the version-field is initialized to an even number. This is
> wrong because at first-time write, field could be any-value.
> 
> Fix simply makes sure that if first-time version-field is odd, increment
> it once more to make it even and only then start standard logic.
> This follows same logic as done in other pvclock shared-pages (See
> kvm_write_wall_clock() and record_steal_time()).
> 
> Signed-off-by: Liran Alon <[e-mail]>
> Reviewed-by: Nikita Leshenko <[e-mail]>
> Reviewed-by: Konrad Rzeszutek Wilk <[e-mail]>
> Signed-off-by: Konrad Rzeszutek Wilk <[e-mail]>
> ---

Applied, thanks.

Wanpeng Li Nov. 13, 2017, 12:40 a.m. UTC | #3

2017-11-05 22:11 GMT+08:00 Liran Alon <[e-mail]>:
> When guest passes KVM it's pvclock-page GPA via WRMSR to
> MSR_KVM_SYSTEM_TIME / MSR_KVM_SYSTEM_TIME_NEW, KVM don't initialize
> pvclock-page to some start-values. It just requests a clock-update which

KVM does, please refer to memset(hv_clock, 0, size) in kvmclock_init().

Regards,
Wanpeng Li

> will happen before entering to guest.
>
> The clock-update logic will call kvm_setup_pvclock_page() to update the
> pvclock-page with info. However, kvm_setup_pvclock_page() *wrongly*
> assumes that the version-field is initialized to an even number. This is
> wrong because at first-time write, field could be any-value.
>
> Fix simply makes sure that if first-time version-field is odd, increment
> it once more to make it even and only then start standard logic.
> This follows same logic as done in other pvclock shared-pages (See
> kvm_write_wall_clock() and record_steal_time()).
>
> Signed-off-by: Liran Alon <[e-mail]>
> Reviewed-by: Nikita Leshenko <[e-mail]>
> Reviewed-by: Konrad Rzeszutek Wilk <[e-mail]>
> Signed-off-by: Konrad Rzeszutek Wilk <[e-mail]>
> ---
>  arch/x86/kvm/x86.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 03869eb7fcd6..181106080e41 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1830,6 +1830,9 @@ static void kvm_setup_pvclock_page(struct kvm_vcpu *v)
>          */
>         BUILD_BUG_ON(offsetof(struct pvclock_vcpu_time_info, version) != 0);
>
> +       if (guest_hv_clock.version & 1)
> +               ++guest_hv_clock.version;  /* first time write, random junk */
> +
>         vcpu->hv_clock.version = guest_hv_clock.version + 1;
>         kvm_write_guest_cached(v->kvm, &vcpu->pv_time,
>                                 &vcpu->hv_clock,
> --
> 1.9.1
>

Liran Alon Nov. 13, 2017, 12:44 a.m. UTC | #4

On 13/11/17 02:40, Wanpeng Li wrote:
> 2017-11-05 22:11 GMT+08:00 Liran Alon <[e-mail]>:
>> When guest passes KVM it's pvclock-page GPA via WRMSR to
>> MSR_KVM_SYSTEM_TIME / MSR_KVM_SYSTEM_TIME_NEW, KVM don't initialize
>> pvclock-page to some start-values. It just requests a clock-update which
>
> KVM does, please refer to memset(hv_clock, 0, size) in kvmclock_init().

kvmclock_init() is the code that runs in KVM-guest. I was talking here 
about the code that handles the WRMSR in KVM hypervisor code.

The issue happens when the guest doesn't init pvclock-page as done in 
kvmclock_init(). Not all guests behave nicely :)

-Liran
>
> Regards,
> Wanpeng Li
>
>> will happen before entering to guest.
>>
>> The clock-update logic will call kvm_setup_pvclock_page() to update the
>> pvclock-page with info. However, kvm_setup_pvclock_page() *wrongly*
>> assumes that the version-field is initialized to an even number. This is
>> wrong because at first-time write, field could be any-value.
>>
>> Fix simply makes sure that if first-time version-field is odd, increment
>> it once more to make it even and only then start standard logic.
>> This follows same logic as done in other pvclock shared-pages (See
>> kvm_write_wall_clock() and record_steal_time()).
>>
>> Signed-off-by: Liran Alon <[e-mail]>
>> Reviewed-by: Nikita Leshenko <[e-mail]>
>> Reviewed-by: Konrad Rzeszutek Wilk <[e-mail]>
>> Signed-off-by: Konrad Rzeszutek Wilk <[e-mail]>
>> ---
>>   arch/x86/kvm/x86.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 03869eb7fcd6..181106080e41 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -1830,6 +1830,9 @@ static void kvm_setup_pvclock_page(struct kvm_vcpu *v)
>>           */
>>          BUILD_BUG_ON(offsetof(struct pvclock_vcpu_time_info, version) != 0);
>>
>> +       if (guest_hv_clock.version & 1)
>> +               ++guest_hv_clock.version;  /* first time write, random junk */
>> +
>>          vcpu->hv_clock.version = guest_hv_clock.version + 1;
>>          kvm_write_guest_cached(v->kvm, &vcpu->pv_time,
>>                                  &vcpu->hv_clock,
>> --
>> 1.9.1
>>

Wanpeng Li Nov. 13, 2017, 12:52 a.m. UTC | #5

2017-11-13 8:44 GMT+08:00 Liran Alon <[e-mail]>:
>
>
> On 13/11/17 02:40, Wanpeng Li wrote:
>>
>> 2017-11-05 22:11 GMT+08:00 Liran Alon <[e-mail]>:
>>>
>>> When guest passes KVM it's pvclock-page GPA via WRMSR to
>>> MSR_KVM_SYSTEM_TIME / MSR_KVM_SYSTEM_TIME_NEW, KVM don't initialize
>>> pvclock-page to some start-values. It just requests a clock-update which
>>
>>
>> KVM does, please refer to memset(hv_clock, 0, size) in kvmclock_init().
>
>
> kvmclock_init() is the code that runs in KVM-guest. I was talking here about
> the code that handles the WRMSR in KVM hypervisor code.
>
> The issue happens when the guest doesn't init pvclock-page as done in
> kvmclock_init(). Not all guests behave nicely :)

But the codes which you modify just works for kvm guest I think.

Regards,
Wanpeng Li

>
> -Liran
>
>>
>> Regards,
>> Wanpeng Li
>>
>>> will happen before entering to guest.
>>>
>>> The clock-update logic will call kvm_setup_pvclock_page() to update the
>>> pvclock-page with info. However, kvm_setup_pvclock_page() *wrongly*
>>> assumes that the version-field is initialized to an even number. This is
>>> wrong because at first-time write, field could be any-value.
>>>
>>> Fix simply makes sure that if first-time version-field is odd, increment
>>> it once more to make it even and only then start standard logic.
>>> This follows same logic as done in other pvclock shared-pages (See
>>> kvm_write_wall_clock() and record_steal_time()).
>>>
>>> Signed-off-by: Liran Alon <[e-mail]>
>>> Reviewed-by: Nikita Leshenko <[e-mail]>
>>> Reviewed-by: Konrad Rzeszutek Wilk <[e-mail]>
>>> Signed-off-by: Konrad Rzeszutek Wilk <[e-mail]>
>>> ---
>>>   arch/x86/kvm/x86.c | 3 +++
>>>   1 file changed, 3 insertions(+)
>>>
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index 03869eb7fcd6..181106080e41 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -1830,6 +1830,9 @@ static void kvm_setup_pvclock_page(struct kvm_vcpu
>>> *v)
>>>           */
>>>          BUILD_BUG_ON(offsetof(struct pvclock_vcpu_time_info, version) !=
>>> 0);
>>>
>>> +       if (guest_hv_clock.version & 1)
>>> +               ++guest_hv_clock.version;  /* first time write, random
>>> junk */
>>> +
>>>          vcpu->hv_clock.version = guest_hv_clock.version + 1;
>>>          kvm_write_guest_cached(v->kvm, &vcpu->pv_time,
>>>                                  &vcpu->hv_clock,
>>> --
>>> 1.9.1
>>>
>

Liran Alon Nov. 13, 2017, 12:55 a.m. UTC | #6

On 13/11/17 02:52, Wanpeng Li wrote:
> 2017-11-13 8:44 GMT+08:00 Liran Alon <[e-mail]>:
>>
>>
>> On 13/11/17 02:40, Wanpeng Li wrote:
>>>
>>> 2017-11-05 22:11 GMT+08:00 Liran Alon <[e-mail]>:
>>>>
>>>> When guest passes KVM it's pvclock-page GPA via WRMSR to
>>>> MSR_KVM_SYSTEM_TIME / MSR_KVM_SYSTEM_TIME_NEW, KVM don't initialize
>>>> pvclock-page to some start-values. It just requests a clock-update which
>>>
>>>
>>> KVM does, please refer to memset(hv_clock, 0, size) in kvmclock_init().
>>
>>
>> kvmclock_init() is the code that runs in KVM-guest. I was talking here about
>> the code that handles the WRMSR in KVM hypervisor code.
>>
>> The issue happens when the guest doesn't init pvclock-page as done in
>> kvmclock_init(). Not all guests behave nicely :)
>
> But the codes which you modify just works for kvm guest I think.
No, it's the code that runs in KVM hypervisor for any guest that knows 
how to work with KVM pv-clock PV interface.
Linux guest is just one of the possible guests which can use this 
interface. kvmclock_init() you mentioned is part of the linux-guest. But 
there are other guests which use KVM pv-clock PV interface.

-Liran
>
> Regards,
> Wanpeng Li
>
>>
>> -Liran
>>
>>>
>>> Regards,
>>> Wanpeng Li
>>>
>>>> will happen before entering to guest.
>>>>
>>>> The clock-update logic will call kvm_setup_pvclock_page() to update the
>>>> pvclock-page with info. However, kvm_setup_pvclock_page() *wrongly*
>>>> assumes that the version-field is initialized to an even number. This is
>>>> wrong because at first-time write, field could be any-value.
>>>>
>>>> Fix simply makes sure that if first-time version-field is odd, increment
>>>> it once more to make it even and only then start standard logic.
>>>> This follows same logic as done in other pvclock shared-pages (See
>>>> kvm_write_wall_clock() and record_steal_time()).
>>>>
>>>> Signed-off-by: Liran Alon <[e-mail]>
>>>> Reviewed-by: Nikita Leshenko <[e-mail]>
>>>> Reviewed-by: Konrad Rzeszutek Wilk <[e-mail]>
>>>> Signed-off-by: Konrad Rzeszutek Wilk <[e-mail]>
>>>> ---
>>>>    arch/x86/kvm/x86.c | 3 +++
>>>>    1 file changed, 3 insertions(+)
>>>>
>>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>>> index 03869eb7fcd6..181106080e41 100644
>>>> --- a/arch/x86/kvm/x86.c
>>>> +++ b/arch/x86/kvm/x86.c
>>>> @@ -1830,6 +1830,9 @@ static void kvm_setup_pvclock_page(struct kvm_vcpu
>>>> *v)
>>>>            */
>>>>           BUILD_BUG_ON(offsetof(struct pvclock_vcpu_time_info, version) !=
>>>> 0);
>>>>
>>>> +       if (guest_hv_clock.version & 1)
>>>> +               ++guest_hv_clock.version;  /* first time write, random
>>>> junk */
>>>> +
>>>>           vcpu->hv_clock.version = guest_hv_clock.version + 1;
>>>>           kvm_write_guest_cached(v->kvm, &vcpu->pv_time,
>>>>                                   &vcpu->hv_clock,
>>>> --
>>>> 1.9.1
>>>>
>>

Wanpeng Li Nov. 13, 2017, 1:03 a.m. UTC | #7

2017-11-13 8:55 GMT+08:00 Liran Alon <[e-mail]>:
>
>
> On 13/11/17 02:52, Wanpeng Li wrote:
>>
>> 2017-11-13 8:44 GMT+08:00 Liran Alon <[e-mail]>:
>>>
>>>
>>>
>>> On 13/11/17 02:40, Wanpeng Li wrote:
>>>>
>>>>
>>>> 2017-11-05 22:11 GMT+08:00 Liran Alon <[e-mail]>:
>>>>>
>>>>>
>>>>> When guest passes KVM it's pvclock-page GPA via WRMSR to
>>>>> MSR_KVM_SYSTEM_TIME / MSR_KVM_SYSTEM_TIME_NEW, KVM don't initialize
>>>>> pvclock-page to some start-values. It just requests a clock-update
>>>>> which
>>>>
>>>>
>>>>
>>>> KVM does, please refer to memset(hv_clock, 0, size) in kvmclock_init().
>>>
>>>
>>>
>>> kvmclock_init() is the code that runs in KVM-guest. I was talking here
>>> about
>>> the code that handles the WRMSR in KVM hypervisor code.
>>>
>>> The issue happens when the guest doesn't init pvclock-page as done in
>>> kvmclock_init(). Not all guests behave nicely :)
>>
>>
>> But the codes which you modify just works for kvm guest I think.
>
> No, it's the code that runs in KVM hypervisor for any guest that knows how
> to work with KVM pv-clock PV interface.
> Linux guest is just one of the possible guests which can use this interface.
> kvmclock_init() you mentioned is part of the linux-guest. But there are
> other guests which use KVM pv-clock PV interface.

Good to know it, could you point one?

>
> -Liran
>
>>
>> Regards,
>> Wanpeng Li
>>
>>>
>>> -Liran
>>>
>>>>
>>>> Regards,
>>>> Wanpeng Li
>>>>
>>>>> will happen before entering to guest.
>>>>>
>>>>> The clock-update logic will call kvm_setup_pvclock_page() to update the
>>>>> pvclock-page with info. However, kvm_setup_pvclock_page() *wrongly*
>>>>> assumes that the version-field is initialized to an even number. This
>>>>> is
>>>>> wrong because at first-time write, field could be any-value.
>>>>>
>>>>> Fix simply makes sure that if first-time version-field is odd,
>>>>> increment
>>>>> it once more to make it even and only then start standard logic.
>>>>> This follows same logic as done in other pvclock shared-pages (See
>>>>> kvm_write_wall_clock() and record_steal_time()).
>>>>>
>>>>> Signed-off-by: Liran Alon <[e-mail]>
>>>>> Reviewed-by: Nikita Leshenko <[e-mail]>
>>>>> Reviewed-by: Konrad Rzeszutek Wilk <[e-mail]>
>>>>> Signed-off-by: Konrad Rzeszutek Wilk <[e-mail]>
>>>>> ---
>>>>>    arch/x86/kvm/x86.c | 3 +++
>>>>>    1 file changed, 3 insertions(+)
>>>>>
>>>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>>>> index 03869eb7fcd6..181106080e41 100644
>>>>> --- a/arch/x86/kvm/x86.c
>>>>> +++ b/arch/x86/kvm/x86.c
>>>>> @@ -1830,6 +1830,9 @@ static void kvm_setup_pvclock_page(struct
>>>>> kvm_vcpu
>>>>> *v)
>>>>>            */
>>>>>           BUILD_BUG_ON(offsetof(struct pvclock_vcpu_time_info, version)
>>>>> !=
>>>>> 0);
>>>>>
>>>>> +       if (guest_hv_clock.version & 1)
>>>>> +               ++guest_hv_clock.version;  /* first time write, random
>>>>> junk */
>>>>> +
>>>>>           vcpu->hv_clock.version = guest_hv_clock.version + 1;
>>>>>           kvm_write_guest_cached(v->kvm, &vcpu->pv_time,
>>>>>                                   &vcpu->hv_clock,
>>>>> --
>>>>> 1.9.1
>>>>>
>>>
>

Wanpeng Li Nov. 13, 2017, 1:37 a.m. UTC | #8

2017-11-13 9:03 GMT+08:00 Wanpeng Li <[e-mail]>:
> 2017-11-13 8:55 GMT+08:00 Liran Alon <[e-mail]>:
>>
>>
>> On 13/11/17 02:52, Wanpeng Li wrote:
>>>
>>> 2017-11-13 8:44 GMT+08:00 Liran Alon <[e-mail]>:
>>>>
>>>>
>>>>
>>>> On 13/11/17 02:40, Wanpeng Li wrote:
>>>>>
>>>>>
>>>>> 2017-11-05 22:11 GMT+08:00 Liran Alon <[e-mail]>:
>>>>>>
>>>>>>
>>>>>> When guest passes KVM it's pvclock-page GPA via WRMSR to
>>>>>> MSR_KVM_SYSTEM_TIME / MSR_KVM_SYSTEM_TIME_NEW, KVM don't initialize
>>>>>> pvclock-page to some start-values. It just requests a clock-update
>>>>>> which
>>>>>
>>>>>
>>>>>
>>>>> KVM does, please refer to memset(hv_clock, 0, size) in kvmclock_init().
>>>>
>>>>
>>>>
>>>> kvmclock_init() is the code that runs in KVM-guest. I was talking here
>>>> about
>>>> the code that handles the WRMSR in KVM hypervisor code.
>>>>
>>>> The issue happens when the guest doesn't init pvclock-page as done in
>>>> kvmclock_init(). Not all guests behave nicely :)
>>>
>>>
>>> But the codes which you modify just works for kvm guest I think.
>>
>> No, it's the code that runs in KVM hypervisor for any guest that knows how
>> to work with KVM pv-clock PV interface.
>> Linux guest is just one of the possible guests which can use this interface.
>> kvmclock_init() you mentioned is part of the linux-guest. But there are
>> other guests which use KVM pv-clock PV interface.
>
> Good to know it, could you point one?

In addition, there is a BUG_ON(if version != 0) just before the codes
which you added. I will move these avoid random junk logics into msr
setup in order that it can avoid '& operation' each time update the pv
stuff. I will send a patch later.

Regards,
Wanpeng Li

Wanpeng Li Nov. 13, 2017, 5:53 a.m. UTC | #9

2017-11-13 9:37 GMT+08:00 Wanpeng Li <[e-mail]>:
> 2017-11-13 9:03 GMT+08:00 Wanpeng Li <[e-mail]>:
>> 2017-11-13 8:55 GMT+08:00 Liran Alon <[e-mail]>:
>>>
>>>
>>> On 13/11/17 02:52, Wanpeng Li wrote:
>>>>
>>>> 2017-11-13 8:44 GMT+08:00 Liran Alon <[e-mail]>:
>>>>>
>>>>>
>>>>>
>>>>> On 13/11/17 02:40, Wanpeng Li wrote:
>>>>>>
>>>>>>
>>>>>> 2017-11-05 22:11 GMT+08:00 Liran Alon <[e-mail]>:
>>>>>>>
>>>>>>>
>>>>>>> When guest passes KVM it's pvclock-page GPA via WRMSR to
>>>>>>> MSR_KVM_SYSTEM_TIME / MSR_KVM_SYSTEM_TIME_NEW, KVM don't initialize
>>>>>>> pvclock-page to some start-values. It just requests a clock-update
>>>>>>> which
>>>>>>
>>>>>>
>>>>>>
>>>>>> KVM does, please refer to memset(hv_clock, 0, size) in kvmclock_init().
>>>>>
>>>>>
>>>>>
>>>>> kvmclock_init() is the code that runs in KVM-guest. I was talking here
>>>>> about
>>>>> the code that handles the WRMSR in KVM hypervisor code.
>>>>>
>>>>> The issue happens when the guest doesn't init pvclock-page as done in
>>>>> kvmclock_init(). Not all guests behave nicely :)
>>>>
>>>>
>>>> But the codes which you modify just works for kvm guest I think.
>>>
>>> No, it's the code that runs in KVM hypervisor for any guest that knows how
>>> to work with KVM pv-clock PV interface.
>>> Linux guest is just one of the possible guests which can use this interface.
>>> kvmclock_init() you mentioned is part of the linux-guest. But there are
>>> other guests which use KVM pv-clock PV interface.
>>
>> Good to know it, could you point one?
>
> In addition, there is a BUG_ON(if version != 0) just before the codes

Oh, not mean version != 0, but I think we can move '& operation' to the setup.

> which you added. I will move these avoid random junk logics into msr
> setup in order that it can avoid '& operation' each time update the pv
> stuff. I will send a patch later.
>
> Regards,
> Wanpeng Li

Patch

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 03869eb7fcd6..181106080e41 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1830,6 +1830,9 @@  static void kvm_setup_pvclock_page(struct kvm_vcpu *v)
 	 */
 	BUILD_BUG_ON(offsetof(struct pvclock_vcpu_time_info, version) != 0);
 
+	if (guest_hv_clock.version & 1)
+		++guest_hv_clock.version;  /* first time write, random junk */
+
 	vcpu->hv_clock.version = guest_hv_clock.version + 1;
 	kvm_write_guest_cached(v->kvm, &vcpu->pv_time,
 				&vcpu->hv_clock,