linux-kernel - Re: [PATCH v3] KVM: x86: do not go through vcpu in __get_kvmclock

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Date:   Thu, 17 Nov 2016 13:14:35 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        rkrcmar@...hat.com
Subject: Re: [PATCH v3] KVM: x86: do not go through vcpu in __get_kvmclock_ns



On 17/11/2016 12:38, Marcelo Tosatti wrote:
> On Wed, Nov 16, 2016 at 06:31:30PM +0100, Paolo Bonzini wrote:
>> Going through the first VCPU is wrong if you follow a KVM_SET_CLOCK with
>> a KVM_GET_CLOCK immediately after, without letting the VCPU run and
>> call kvm_guest_time_update.
>>
>> To fix this, compute the kvmclock value ourselves, using the master
>> clock (tsc, nsec) pair as the base and the host CPU frequency as
>> the scale.
>>
>> Reported-by: Marcelo Tosatti <mtosatti@...hat.com>
>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
>> ---
>>  arch/x86/kvm/x86.c | 21 +++++++++++++--------
>>  1 file changed, 13 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 1ba08278a9a9..bd138a79404a 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -1724,18 +1724,23 @@ static void kvm_gen_update_masterclock(struct kvm *kvm)
>>  
>>  static u64 __get_kvmclock_ns(struct kvm *kvm)
>>  {
>> -	struct kvm_vcpu *vcpu = kvm_get_vcpu(kvm, 0);
>>  	struct kvm_arch *ka = &kvm->arch;
>> -	s64 ns;
>> +	struct pvclock_vcpu_time_info hv_clock;
>>  
>> -	if (vcpu->arch.hv_clock.flags & PVCLOCK_TSC_STABLE_BIT) {
>> -		u64 tsc = kvm_read_l1_tsc(vcpu, rdtsc());
>> -		ns = __pvclock_read_cycles(&vcpu->arch.hv_clock, tsc);
>> -	} else {
>> -		ns = ktime_get_boot_ns() + ka->kvmclock_offset;
>> +	spin_lock(&ka->pvclock_gtod_sync_lock);
>> +	if (!ka->use_master_clock) {
>> +		spin_unlock(&ka->pvclock_gtod_sync_lock);
>> +		return ktime_get_boot_ns() + ka->kvmclock_offset;
>>  	}
>>  
>> -	return ns;
>> +	hv_clock.tsc_timestamp = ka->master_cycle_now;
>> +	hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset;
>> +	spin_unlock(&ka->pvclock_gtod_sync_lock);
>> +
>> +	kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
>> +			   &hv_clock.tsc_shift,
>> +			   &hv_clock.tsc_to_system_mul);
>> +	return __pvclock_read_cycles(&hv_clock, rdtsc());
>>  }
> 
> Missing TSC scaling? 
> 
>         /* With all the info we got, fill in the values */
> 
>         if (kvm_has_tsc_control)
>                 tgt_tsc_khz = kvm_scale_tsc(v, tgt_tsc_khz);
> 
> Should use kvm_read_l1_tsc to convert as well?

We don't have a vcpu, so we cannot use either kvm_scale_tsc or
kvm_read_l1_tsc.  But luckily it's not necessary here, because we're
using cpu_tsc_khz and rdtsc.

Paolo