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

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Date:   Thu, 17 Nov 2016 09:38:19 -0200
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Paolo Bonzini <pbonzini@...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 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?