On 8/21/25 21:46, Xiaoyao Li wrote: > On 8/22/2025 3:43 AM, Sohil Mehta wrote: >> On 8/21/2025 12:34 PM, Sohil Mehta wrote: >>> On 8/21/2025 6:15 AM, David Woodhouse wrote: >>> >>>> Hm. 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. > > yeah. It's the intentional behavior of QEMU. > > Invariant TSC is ummigratable unless users explicitly configures the TSC > frequency, e.g., "-cpu host,tsc-frequency=xxx". Because the TSC > frequency is by default the host's frequency if no "tsc-frequency" > specified, and it will change when the VM is migrated to a host with a > different TSC frequency. > > It's the specific behavior/rule of QEMU. We just need to keep it in > mind. If we want to expose invariant TSC to the guest with QEMU's "-cpu > host", we can either: > 1) explicitly configure the "tsc-frequency", or > 2) explicitly turn off "migratable"
Could the TSC frequency be included in the migration stream? -- Sincerely, Demi Marie Obenour (she/her/hers)
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