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The sad state of TSC being notoriously non-sychronized for several decades caused the kernel to grow quite rigorous sanity checks to detect whether the TSC is valid to be used for timekeeping. The TSC ADJUST MSR provides the offset between the initial TSC value after hardware reset and later modifications. This allows to detect cases where firmware tampers with the TSC and also allows to correct the firmware induced damage by resetting the offset in a controlled way. The universal correct rule is that the TSC ADJUST value has to be consistent within all CPUs of a socket. The kernel further assumes that the TSC offset should be consistent between sockets. That's not really correct as systems with a huge number of sockets are not architecurally guaranteed to reset the per socket TSC base synchronously. In case that the per socket offset is not consistent the kernel resets it to the offset of the boot CPU and then does a synchronization check which corrects for the inter socket delays. That works most of the time, but it is suboptimal as the firmware has eventually better information about the per socket offset and on sane systems that offset should just work in the validation checks. -----BEGIN PGP SIGNATURE----- iQJHBAABCgAxFiEEQp8+kY+LLUocC4bMphj1TA10mKEFAmZCDA4THHRnbHhAbGlu dXRyb25peC5kZQAKCRCmGPVMDXSYodLcD/4hZmCiEL97M+qb0rjmscKmJq/EOjxm pRgT/+vH2MakYh2xIjLSeMtRB5eFdfz+ZspJGEt017yW5l+saZ6edrq+g2qi1EfJ TDGbDGK9T6HR0WDplFLqLXolKS2lcvHbolATu/t5ZQmrRmuGuS+6t6eAoI9QcaWQ DaqMtdSQNq8B5hopaZtaJSTTkznD/CtKyMCvVKGxXE2gH6d6UmezR72f6oruzgg9 WXLDt1sPxg1zl1rS1GdeRa4xXrsLxr8THZ53Nr5pPyZV6FCSOZQtcurwhsIYcMO7 b3m+LU04XGURK196c0Uej8UwRCAHpD50aS91GcclXsR4wTKyFatVz9mpwZOK/F/L Pw+5O6xUeyIAKMr6YJl6KusPhhwDcYm+ETuTzMmWMyJEh91lLYHyCKniE3wsHpzT L7er6HWOwBaPlOnvuOhl4rzqr0F+9xLmWWq6s+85HlvlgfV1NjEhqi7dn/ZO1jdx Im3Xq8sq04tIMNjLPSTkovXmvU2us45yQk2HthWSM7FQ+vpzPDgdp1sVFsLK19cu +t0jI01qSUBzYvcM28CDX99hEI2L8Oo/nC1/Vq/4MB+KkEPCUMKr0ZA2nTKHL8lx +lOGdnzokr6DsbRdtfqWKywc/is2r1OXrOSBR23SwwK1XQ2aRMi/0F96Q0cR9lzt 6utxZRFhc7BtpA== =W1qL -----END PGP SIGNATURE----- Merge tag 'x86-timers-2024-05-13' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip Pull x86 timers update from Thomas Gleixner: "A single update for the TSC synchronixation sanity checks: The sad state of TSC being notoriously non-sychronized for several decades caused the kernel to grow quite rigorous sanity checks to detect whether the TSC is valid to be used for timekeeping. The TSC ADJUST MSR provides the offset between the initial TSC value after hardware reset and later modifications. This allows to detect cases where firmware tampers with the TSC and also allows to correct the firmware induced damage by resetting the offset in a controlled way. The universal correct rule is that the TSC ADJUST value has to be consistent within all CPUs of a socket. The kernel further assumes that the TSC offset should be consistent between sockets. That's not really correct as systems with a huge number of sockets are not architecurally guaranteed to reset the per socket TSC base synchronously. In case that the per socket offset is not consistent the kernel resets it to the offset of the boot CPU and then does a synchronization check which corrects for the inter socket delays. That works most of the time, but it is suboptimal as the firmware has eventually better information about the per socket offset and on sane systems that offset should just work in the validation checks" * tag 'x86-timers-2024-05-13' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: x86/tsc: Trust initial offset in architectural TSC-adjust MSRs
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 reStructuredText 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.