Re: [PATCH 1/2] KVM: x86: implement KVM_SET_TSC_PRECISE/KVM_GET_TSC_PRECISE

[Paolo Bonzini]

On 30/11/20 14:35, Maxim Levitsky wrote:
> +		if (guest_cpuid_has(vcpu, X86_FEATURE_TSC_ADJUST)) {
> +			tsc_state.tsc_adjust = vcpu->arch.ia32_tsc_adjust_msr;
> +			tsc_state.flags |= KVM_TSC_STATE_TSC_ADJUST_VALID;
> +		}

This is mostly useful for userspace that doesn't disable the quirk, right?

> +		kvm_get_walltime(&wall_nsec, &host_tsc);
> +		diff = wall_nsec - tsc_state.nsec;
> +
> +		if (diff < 0 || tsc_state.nsec == 0)
> +			diff = 0;
> +

diff < 0 should be okay.  Also why the nsec==0 special case?  What about 
using a flag instead?

Paolo