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Thu, 21 Aug 2025 19:43:26 +0000 Message-ID: <5f5f1230-f373-469c-b0d9-abc80199886e@intel.com> Date: Thu, 21 Aug 2025 12:43:21 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 13/15] x86/cpu/intel: Bound the non-architectural constant_tsc model checks Content-Language: en-US From: Sohil Mehta To: David Woodhouse , , Dave Hansen , Tony Luck , =?UTF-8?Q?J=C3=BCrgen_Gross?= , Boris Ostrovsky , xen-devel CC: Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Namhyung Kim , Mark Rutland , "Alexander Shishkin" , Jiri Olsa , Ian Rogers , Adrian Hunter , "Kan Liang" , Thomas Gleixner , Borislav Petkov , "H . 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My test host is INTEL_HASWELL_X (0x63f). For reasons which are >> unclear to me, QEMU doesn't set bit 8 of 0x80000007 EDX unless I >> explicitly append ',+invtsc' to the existing '-cpu host' on its command >> line. So now my guest doesn't think it has X86_FEATURE_CONSTANT_TSC. >> > > Haswell should have X86_FEATURE_CONSTANT_TSC, so I would have expected > the guest bit to be set. Until now, X86_FEATURE_CONSTANT_TSC was set > based on the Family-model instead of the CPUID enumeration which may > have hid the issue. > Correction: s/instead/as well as > From my initial look at the QEMU implementation, this seems intentional. > > QEMU considers Invariant TSC as un-migratable which prevents it from > being exposed to migratable guests (default). > target/i386/cpu.c: > [FEAT_8000_0007_EDX] > .unmigratable_flags = CPUID_APM_INVTSC, > > Can you please try '-cpu host,migratable=off'? This is mainly to verify. If confirmed, I am not sure what the long term solution should be.