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8b0e00fba9
On certain CPUs, Linux guests expect HWCR.TscFreqSel[bit 24] to be set. If it isn't set, they complain: [Firmware Bug]: TSC doesn't count with P0 frequency! Allow userspace (and the guest) to set this bit in the virtual HWCR to eliminate the above complaint. Allow the guest to write the bit even though its is R/O on *some* CPUs. Like many bits in HWRC, TscFreqSel is not architectural at all. On Family 10h[1], it was R/W and powered on as 0. In Family 15h, one of the "changes relative to Family 10H Revision D processors[2] was: • MSRC001_0015 [Hardware Configuration (HWCR)]: • Dropped TscFreqSel; TSC can no longer be selected to run at NB P0-state. Despite the "Dropped" above, that same document later describes HWCR[bit 24] as follows: TscFreqSel: TSC frequency select. Read-only. Reset: 1. 1=The TSC increments at the P0 frequency If the guest clears the bit, the worst case scenario is the guest will be no worse off than it is today, e.g. the whining may return after a guest clears the bit and kexec()'s into a new kernel. [1] https://www.amd.com/content/dam/amd/en/documents/archived-tech-docs/programmer-references/31116.pdf [2] https://www.amd.com/content/dam/amd/en/documents/archived-tech-docs/programmer-references/42301_15h_Mod_00h-0Fh_BKDG.pdf, Signed-off-by: Jim Mattson <jmattson@google.com> Link: https://lore.kernel.org/r/20230929230246.1954854-3-jmattson@google.com [sean: elaborate on why the bit is writable by the guest] Signed-off-by: Sean Christopherson <seanjc@google.com> |
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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.