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

[Wanpeng Li <kernellwp@gmail.com>]

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.

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
>>>
>