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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id w3sm9211804edt.84.2020.11.30.06.33.47 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 30 Nov 2020 06:33:47 -0800 (PST) Subject: Re: [PATCH 1/2] KVM: x86: implement KVM_SET_TSC_PRECISE/KVM_GET_TSC_PRECISE To: Maxim Levitsky , kvm@vger.kernel.org Cc: Oliver Upton , Ingo Molnar , Sean Christopherson , Thomas Gleixner , open list , Marcelo Tosatti , Jonathan Corbet , Wanpeng Li , Borislav Petkov , Jim Mattson , "H. Peter Anvin" , "open list:DOCUMENTATION" , Joerg Roedel , "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , Vitaly Kuznetsov References: <20201130133559.233242-1-mlevitsk@redhat.com> <20201130133559.233242-2-mlevitsk@redhat.com> From: Paolo Bonzini 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 List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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