linux-kernel - Re: [PATCH 1/2] KVM: x86: implement KVM_SET_TSC_PRECISE/KVM_GET_TSC

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Date:   Mon, 30 Nov 2020 15:33:46 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     Oliver Upton <oupton@...gle.com>, Ingo Molnar <mingo@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        open list <linux-kernel@...r.kernel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>,
        Jim Mattson <jmattson@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH 1/2] KVM: x86: implement
 KVM_SET_TSC_PRECISE/KVM_GET_TSC_PRECISE

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