Re: [PATCH] KVM: x86: pvclock: Handle first-time write to pvclock-page contains random junk

[Wanpeng Li <kernellwp@gmail.com>]

2017-11-13 8:55 GMT+08:00 Liran Alon <LIRAN.ALON@oracle.com>:
>
>
> On 13/11/17 02:52, Wanpeng Li wrote:
>>
>> 2017-11-13 8:44 GMT+08:00 Liran Alon <LIRAN.ALON@oracle.com>:
>>>
>>>
>>>
>>> On 13/11/17 02:40, Wanpeng Li wrote:
>>>>
>>>>
>>>> 2017-11-05 22:11 GMT+08:00 Liran Alon <liran.alon@oracle.com>:
>>>>>
>>>>>
>>>>> 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 <liran.alon@oracle.com>
>>>>> Reviewed-by: Nikita Leshenko <nikita.leshchenko@oracle.com>
>>>>> Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>>>>> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>>>>> ---
>>>>>    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
>>>>>
>>>
>