From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-10.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 304E5C433DB for ; Wed, 31 Mar 2021 10:00:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 07DD6619A9 for ; Wed, 31 Mar 2021 10:00:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234932AbhCaJ7t (ORCPT ); Wed, 31 Mar 2021 05:59:49 -0400 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:47615 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234629AbhCaJ7c (ORCPT ); Wed, 31 Mar 2021 05:59:32 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1617184771; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=qsqxfwVHrmfsyTMmPvqiQy5Due+Xql9QpbIFHBdT44A=; b=cMFbnZuz/ESgPq/ovKxGqCyoH4WlD2AWyHz2BVoIWW1T4SQXEMp9Y0HXAklL/F+HqLrzUL W4NCDnrA/AwthUExMRg3WFs/gnROdidI5Yk4iqJcTWLxDDTNvvnyVfffamW3s9NJv+m3HG JvfDU5KvvQabISvY43HmcjMT/HXKarw= Received: from mail-ed1-f70.google.com (mail-ed1-f70.google.com [209.85.208.70]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-602-pgsyjQ6-POiuqT_BYe2K_Q-1; Wed, 31 Mar 2021 05:59:29 -0400 X-MC-Unique: pgsyjQ6-POiuqT_BYe2K_Q-1 Received: by mail-ed1-f70.google.com with SMTP id t27so839994edi.2 for ; Wed, 31 Mar 2021 02:59:29 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:in-reply-to:references:date :message-id:mime-version; bh=qsqxfwVHrmfsyTMmPvqiQy5Due+Xql9QpbIFHBdT44A=; b=OscDdTvMepX4UUuEbEEZJvIJi0xW13InTictmkmzdQr015znh3lqvYymQZw6nr3U1h pCK1EHOSEmJivSBfgMNjb6Eqhhi5NSZJaITbu65Z5bQlJFYBGjLhg+cVzSvNn2VLiaT0 7/h+ffvCxuZUx7hPF54rMtAtWjgRvry0yRM68SGDXxdl4bqyMB/MNYA41dOKkD6lq7cH PhEuWo1XJifKpquXm1y6xGbw2VAa6GEZ0KaBbVM64EaTnyqBkzUsbpXxTqb9G+5B9GLM 1ahq8AbbYPEMa1ok5gH18zjK3Af7O2v5oSbNrJNSoPOXUjprDtVZplAqdptDANeohFbC jANA== X-Gm-Message-State: AOAM530Ks+e/8YwDNmMgODMTrQD3IiR1sNazX//FKAmoltjvVtkwSsoc xESzyWWaqok2UdFU/M2+em3FFT71f2zPldg1JsmlvdVZjnQLmqlygJX0Q+dzEOsQAOQO8gS6zQQ Xo+WuOaro2X0A X-Received: by 2002:a17:906:9714:: with SMTP id k20mr2576837ejx.519.1617184768792; Wed, 31 Mar 2021 02:59:28 -0700 (PDT) X-Google-Smtp-Source: ABdhPJzzwl88y+LANJ8ILEZrk/v4Wj3wP1EQ+m2kd5oggG1DBZv1vJGMPUQhCP9KJR1ZEVYELwNj1w== X-Received: by 2002:a17:906:9714:: with SMTP id k20mr2576831ejx.519.1617184768636; Wed, 31 Mar 2021 02:59:28 -0700 (PDT) Received: from vitty.brq.redhat.com (g-server-2.ign.cz. [91.219.240.2]) by smtp.gmail.com with ESMTPSA id cy5sm1173596edb.46.2021.03.31.02.59.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 31 Mar 2021 02:59:28 -0700 (PDT) From: Vitaly Kuznetsov To: Paolo Bonzini , Marcelo Tosatti Cc: kvm@vger.kernel.org, Sean Christopherson , Wanpeng Li , Jim Mattson Subject: Re: [PATCH v2 1/2] KVM: x86: hyper-v: Properly divide maybe-negative 'hv_clock->system_time' in compute_tsc_page_parameters() In-Reply-To: <7c6f61e9-f2a6-46dc-7ab6-dc6ae86c7b60@redhat.com> References: <20210329114800.164066-1-vkuznets@redhat.com> <20210329114800.164066-2-vkuznets@redhat.com> <20210330131236.GA5932@fuller.cnet> <87ft0cu2eq.fsf@vitty.brq.redhat.com> <4d7f375c-c912-fbeb-edd1-03d742d49dcb@redhat.com> <87a6qju97s.fsf@vitty.brq.redhat.com> <7c6f61e9-f2a6-46dc-7ab6-dc6ae86c7b60@redhat.com> Date: Wed, 31 Mar 2021 11:59:27 +0200 Message-ID: <871rbvtzi8.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org Paolo Bonzini writes: > On 31/03/21 08:29, Vitaly Kuznetsov wrote: >> I'm leaning towards making a v3 and depending on how complex it's going >> to look like we can decide which way to go. > > As you prefer, but I would have no problem with committing v2 for now. > From the point of view of system_time being a signed delta your v2 is > not a regression. Ok, I convinced myself this is correct: @@ -5744,8 +5745,22 @@ long kvm_arch_vm_ioctl(struct file *filp, * pvclock_update_vm_gtod_copy(). */ kvm_gen_update_masterclock(kvm); - now_ns = get_kvmclock_ns(kvm); - kvm->arch.kvmclock_offset += user_ns.clock - now_ns; + + /* + * This pairs with kvm_guest_time_update(): when masterclock is + * in use, we use master_kernel_ns + kvmclock_offset to set + * unsigned 'system_time' so if we use get_kvmclock_ns() (which + * is slightly ahead) here we risk going negative on unsigned + * 'system_time' when 'user_ns.clock' is very small. + */ + spin_lock(&ka->pvclock_gtod_sync_lock); + if (kvm->arch.use_master_clock) + now_ns = ka->master_kernel_ns; + else + now_ns = get_kvmclock_base_ns(); + ka->kvmclock_offset = user_ns.clock - now_ns; + spin_unlock(&ka->pvclock_gtod_sync_lock); + kvm_make_all_cpus_request(kvm, KVM_REQ_CLOCK_UPDATE); break; } In !masterclock case kvm_guest_time_update() uses get_kvmclock_base_ns() (and get_kvmclock_ns() is just get_kvmclock_base_ns() + ka->kvmclock_offset) so we're good. Also, it looks like a good idea to put the whole calculation under spinlock here. Personally, I like this a little bit more than treating 'system_time' as signed, what do you guys think? -- Vitaly