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"
