Re: KVM: x86: workaround SuSE's 2.6.16 pvclock vs masterclock issue

[Paolo Bonzini <pbonzini@redhat.com>]

On 20/01/2015 18:54, Marcelo Tosatti wrote:
> 
> SuSE's 2.6.16 kernel fails to boot if the delta between tsc_timestamp
> and rdtsc is larger than a given threshold:
> 
>  * If we get more than the below threshold into the future, we rerequest
>  * the real time from the host again which has only little offset then
>  * that we need to adjust using the TSC.
>  *
>  * For now that threshold is 1/5th of a jiffie. That should be good
>  * enough accuracy for completely broken systems, but also give us swing
>  * to not call out to the host all the time.
>  */
> #define PVCLOCK_DELTA_MAX ((1000000000ULL / HZ) / 5)
> 
> Disable masterclock support (which increases said delta) in case the
> boot vcpu does not use MSR_KVM_SYSTEM_TIME_NEW.

Makes sense.

Applied to queue, thanks.

Paolo

> Upstreams kernels which support pvclock vsyscalls (and therefore make
> use of PVCLOCK_STABLE_BIT) use MSR_KVM_SYSTEM_TIME_NEW.
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 7c492ed..9a099f6 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -613,6 +613,8 @@ struct kvm_arch {
>  	#ifdef CONFIG_KVM_MMU_AUDIT
>  	int audit_point;
>  	#endif
> +
> +	bool boot_vcpu_runs_old_kvmclock;
>  };
>  
>  struct kvm_vm_stat {
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8f1e22d..1d8a4f6 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1494,7 +1494,8 @@ static void pvclock_update_vm_gtod_copy(struct kvm *kvm)
>  					&ka->master_cycle_now);
>  
>  	ka->use_master_clock = host_tsc_clocksource && vcpus_matched
> -				&& !backwards_tsc_observed;
> +				&& !backwards_tsc_observed
> +				&& !ka->boot_vcpu_runs_old_kvmclock;
>  
>  	if (ka->use_master_clock)
>  		atomic_set(&kvm_guest_has_master_clock, 1);
> @@ -2106,8 +2107,20 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  	case MSR_KVM_SYSTEM_TIME_NEW:
>  	case MSR_KVM_SYSTEM_TIME: {
>  		u64 gpa_offset;
> +		struct kvm_arch *ka = &vcpu->kvm->arch;
> +
>  		kvmclock_reset(vcpu);
>  
> +		if (vcpu->vcpu_id == 0 && !msr_info->host_initiated) {
> +			bool tmp = (msr == MSR_KVM_SYSTEM_TIME);
> +
> +			if (ka->boot_vcpu_runs_old_kvmclock != tmp)
> +				set_bit(KVM_REQ_MASTERCLOCK_UPDATE,
> +					&vcpu->requests);
> +
> +			ka->boot_vcpu_runs_old_kvmclock = tmp;
> +		}
> +
>  		vcpu->arch.time = data;
>  		kvm_make_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu);
>  
>