From mboxrd@z Thu Jan 1 00:00:00 1970 From: Prarit Bhargava Subject: Re: [PATCH] x86, clock: Fix kvm guest tsc initialization Date: Thu, 08 Sep 2016 09:46:29 -0400 Message-ID: <57D16BB5.5050501@redhat.com> References: <1473340038-26432-1-git-send-email-prarit@redhat.com> <67c4faca-5c32-85de-10c2-59eff3c9fb87@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Cc: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , x86@kernel.org, Len Brown , "Peter Zijlstra (Intel)" , Borislav Petkov , Adrian Hunter , "Christopher S. Hall" , David Woodhouse , kvm@vger.kernel.org To: Paolo Bonzini , linux-kernel@vger.kernel.org Return-path: In-Reply-To: <67c4faca-5c32-85de-10c2-59eff3c9fb87@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 09/08/2016 09:33 AM, Paolo Bonzini wrote: > > > On 08/09/2016 15:07, Prarit Bhargava wrote: >> When booting a kvm guest on AMD with the latest kernel the following >> messages are displayed in the boot log: >> >> tsc: Unable to calibrate against PIT >> tsc: HPET/PMTIMER calibration failed >> >> aa297292d708 ("x86/tsc: Enumerate SKL cpu_khz and tsc_khz via CPUID") >> introduced a change to account for a difference in cpu and tsc frequencies for >> Intel SKL processors. Before this change the native tsc set >> x86_platform.calibrate_tsc to native_calibrate_tsc() which is a hardware >> calibration of the tsc, and in tsc_init() executed >> >> tsc_khz = x86_platform.calibrate_tsc(); >> cpu_khz = tsc_khz; >> >> The kvm code changed x86_platform.calibrate_tsc to kvm_get_tsc_khz() and >> executed the same tsc_init() function. This meant that KVM guests did not >> execute the native hardware calibration function. >> >> After aa297292d708, there are separate native calibrations for cpu_khz and >> tsc_khz. The code sets x86_platform.calibrate_tsc to native_calibrate_tsc() >> which is now an Intel specific calibration function , and >> x86_platform.calibrate_cpu to native_calibrate_cpu() which is the "old" >> native_calibrate_tsc() function (ie, the native hardware calibration >> function). [...] >> >> The kvm code should not call the hardware initialization in >> native_calibrate_cpu(), as it isn't applicable for kvm and it didn't do that >> prior to aa297292d708. Setting x86_platform.calibrate_cpu to NULL is not >> appropriate as cpu_khz_from_cpuid() must be called to get the correct >> value of cpu_khz on Intel KVM guests. >> >> This patch resolves this issue by setting x86_platform.calibrate_cpu to >> cpu_khz_from_cpuid() for KVM guests, which allows Intel KVM guests to get >> the right cpu frequency. > > KVM guests don't have that CPUID leaf at all. kvm_get_tsc_khz can > double as x86_platform.calibrate_cpu too for KVM guests, restoring the > behavior prior to aa297292d708. Thanks Paolo -- v2 coming shortly. P.