Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH 1/2] KVM: x86: implement
 KVM_SET_TSC_PRECISE/KVM_GET_TSC_PRECISE
To:     Maxim Levitsky <mlevitsk@redhat.com>, kvm@vger.kernel.org
Cc:     Oliver Upton <oupton@google.com>, Ingo Molnar <mingo@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        open list <linux-kernel@vger.kernel.org>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Wanpeng Li <wanpengli@tencent.com>,
        Borislav Petkov <bp@alien8.de>,
        Jim Mattson <jmattson@google.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Joerg Roedel <joro@8bytes.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        Vitaly Kuznetsov <vkuznets@redhat.com>
References: <20201130133559.233242-1-mlevitsk@redhat.com>
 <20201130133559.233242-2-mlevitsk@redhat.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <38602ef4-7ecf-a5fd-6db9-db86e8e974e4@redhat.com>
Date:   Mon, 30 Nov 2020 15:33:46 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.4.0
MIME-Version: 1.0
In-Reply-To: <20201130133559.233242-2-mlevitsk@redhat.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk

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