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Don't snapshot tsc_khz into per-cpu cpu_tsc_khz if the host TSC is constant, in which case the actual TSC frequency will never change and thus capturing TSC during initialization is unnecessary, KVM can simply use tsc_khz. This value is snapshotted from kvm_timer_init->kvmclock_cpu_online->tsc_khz_changed(NULL) On CPUs with constant TSC, but not a hardware-specified TSC frequency, snapshotting cpu_tsc_khz and using that to set a VM's target TSC frequency can lead to VM to think its TSC frequency is not what it actually is if refining the TSC completes after KVM snapshots tsc_khz. The actual frequency never changes, only the kernel's calculation of what that frequency is changes. Ideally, KVM would not be able to race with TSC refinement, or would have a hook into tsc_refine_calibration_work() to get an alert when refinement is complete. Avoiding the race altogether isn't practical as refinement takes a relative eternity; it's deliberately put on a work queue outside of the normal boot sequence to avoid unnecessarily delaying boot. Adding a hook is doable, but somewhat gross due to KVM's ability to be built as a module. And if the TSC is constant, which is likely the case for every VMX/SVM-capable CPU produced in the last decade, the race can be hit if and only if userspace is able to create a VM before TSC refinement completes; refinement is slow, but not that slow. For now, punt on a proper fix, as not taking a snapshot can help some uses cases and not taking a snapshot is arguably correct irrespective of the race with refinement. Signed-off-by: Anton Romanov <romanton@google.com> Reviewed-by: Sean Christopherson <seanjc@google.com> Link: https://lore.kernel.org/r/20220608183525.1143682-1-romanton@google.com Signed-off-by: Sean Christopherson <seanjc@google.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.