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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9E334E8FDB3 for ; Tue, 3 Oct 2023 21:00:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232285AbjJCVAR (ORCPT ); Tue, 3 Oct 2023 17:00:17 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54468 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232120AbjJCVAP (ORCPT ); Tue, 3 Oct 2023 17:00:15 -0400 Received: from mail-yb1-xb4a.google.com (mail-yb1-xb4a.google.com [IPv6:2607:f8b0:4864:20::b4a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 33903AC for ; Tue, 3 Oct 2023 14:00:09 -0700 (PDT) Received: by mail-yb1-xb4a.google.com with SMTP id 3f1490d57ef6-d8660e23801so1690553276.3 for ; Tue, 03 Oct 2023 14:00:09 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1696366808; x=1696971608; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=mE7WbSRGG3IBXYNyxwgRAed3lO7KyWMV50CsUdXZcYc=; b=TqutrsOmn9atFkU2oGu/uezbxygdBlh0cVfTPBgn7AnUDjklWdqjlycZiIZN0mRgH9 Vb6LvB/nzvC2vYLCYAmbOfdWvT2vUSQkOWbdqykKWC29xyVQg4uLG/e1dxhRQmniZy5t ChI0pD7cM1iNITN5nyj3GdDrpkVANEnrpr6MiEvaYw3eyUN8Wq77RaeITIa725AHPjBB tKVgZMsvK6afuDJTJOGw9PKQwCKmUfacfjofLKW/4a+XG7DW5neDya6iTiisPoF0/RQk sK+Gww8pi+LfWt1uXkP4eeLRhSyJhsB/qmYq5B+j9gEOeMFUvbcMKRo0+vGKRPDHh3cV Yg2g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1696366808; x=1696971608; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=mE7WbSRGG3IBXYNyxwgRAed3lO7KyWMV50CsUdXZcYc=; b=PKs52pQ8x0f3ZPv/ZDpXlgrV04yCpDV4KW9dWW8tLHr74F+Mp7U4juAu8QX4tyS/9O X1jLTE+Ks/NyWtVC8+++NDGsWJtxk0AQ0GriwFV86dX1quNGEFVbojsp2Zwe9YkNA8pe gbesnxOwOvG03R1Ayg0jqu+sskfr+CxQRuvuffI7gSG4mFB+6XEDSJ1/e4nUmC13rLo3 OxYjd9VHaiinH0nXxoCSgvgqSpW9xJIalVxk97i2I7i4hr0SkWbX8Z+V1Lcqzq/V795k E5POq4C6AiqOzuRG0JCwj3WjlQH+bTCVYtXlg7NU3aplJu/sQMfr+MWfOurNz0NtqPcW fDGg== X-Gm-Message-State: AOJu0YwUCiel1VBaFiQqi43QG8lmy1pdrSGAxiP1liFld3xBi+onB758 W1e/m3+xoVkArsIKMJMcujwXcEy5RGU= X-Google-Smtp-Source: AGHT+IGbJxXZ4vvK5lEPewhlyhfI4WFnbA1LXX3nxv5zgeQeix4PkOW5F84jEuW4er/V5aRynVB7EGLG54c= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a25:482:0:b0:d81:b2db:8c2e with SMTP id 124-20020a250482000000b00d81b2db8c2emr5346ybe.6.1696366808381; Tue, 03 Oct 2023 14:00:08 -0700 (PDT) Date: Tue, 3 Oct 2023 14:00:06 -0700 In-Reply-To: <1326f47a-45c0-963d-d50f-a9774d932744@oracle.com> Mime-Version: 1.0 References: <36f3dbb1-61d7-e90a-02cf-9f151a1a3d35@oracle.com> <884aa233ef46d5209b2d1c92ce992f50a76bd656.camel@infradead.org> <52a3cea2084482fc67e35a0bf37453f84dcd6297.camel@infradead.org> <1326f47a-45c0-963d-d50f-a9774d932744@oracle.com> Message-ID: Subject: Re: [PATCH RFC 1/1] KVM: x86: add param to update master clock periodically From: Sean Christopherson To: Dongli Zhang Cc: David Woodhouse , Joe Jin , x86@kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, pbonzini@redhat.com, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com Content-Type: text/plain; charset="us-ascii" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Oct 02, 2023, Dongli Zhang wrote: > Hi Sean, > > On 10/2/23 18:49, Sean Christopherson wrote: > > On Mon, Oct 02, 2023, Dongli Zhang wrote: > >>> @@ -12185,6 +12203,10 @@ int kvm_arch_hardware_enable(void) > >>> if (ret != 0) > >>> return ret; > >>> > >>> + if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC)) > >>> + kvm_get_time_scale(NSEC_PER_SEC, tsc_khz * 1000LL, > >>> + &host_tsc_shift, &host_tsc_to_system_mul); > >> > >> I agree that to use the kvmclock to calculate the ns elapsed when updating the > >> master clock. > >> > >> Would you take the tsc scaling into consideration? > >> > >> While the host_tsc_shift and host_tsc_to_system_mul are pre-computed, how about > >> the VM using different TSC frequency? > > > > Heh, I'm pretty sure that's completely broken today. I don't see anything in KVM > > that takes hardware TSC scaling into account. > > > > This code: > > > > if (unlikely(vcpu->hw_tsc_khz != tgt_tsc_khz)) { > > kvm_get_time_scale(NSEC_PER_SEC, tgt_tsc_khz * 1000LL, > > &vcpu->hv_clock.tsc_shift, > > &vcpu->hv_clock.tsc_to_system_mul); > > vcpu->hw_tsc_khz = tgt_tsc_khz; > > kvm_xen_update_tsc_info(v); > > } > > > > is recomputing the multipler+shift for the current *physical* CPU, it's not > > related to the guest's TSC in any way. > > The below is the code. > > line 3175: query freq for current *physical* CPU. > > line 3211: scale the freq if scaling is involved. > > line 3215: compute the view for guest based on new 'tgt_tsc_khz' after scaling. > > 3146 static int kvm_guest_time_update(struct kvm_vcpu *v) > 3147 { > 3148 unsigned long flags, tgt_tsc_khz; > 3149 unsigned seq; > ... ... > 3173 /* Keep irq disabled to prevent changes to the clock */ > 3174 local_irq_save(flags); > 3175 tgt_tsc_khz = get_cpu_tsc_khz(); > ... ... > 3210 if (kvm_caps.has_tsc_control) > 3211 tgt_tsc_khz = kvm_scale_tsc(tgt_tsc_khz, > 3212 v->arch.l1_tsc_scaling_ratio); > 3213 > 3214 if (unlikely(vcpu->hw_tsc_khz != tgt_tsc_khz)) { > 3215 kvm_get_time_scale(NSEC_PER_SEC, tgt_tsc_khz * 1000LL, > 3216 &vcpu->hv_clock.tsc_shift, > 3217 &vcpu->hv_clock.tsc_to_system_mul); > 3218 vcpu->hw_tsc_khz = tgt_tsc_khz; > 3219 kvm_xen_update_tsc_info(v); > 3220 } > > > Would you please let me know if the above understanding is incorrect? Ah, yeah, you're correct. I missed the call to kvm_scale_tsc() at 3211.