caea091e48
Kvm elects to use tsc instead of kvm-clock when it can detect that the
TSC is invariant.
(As of commit 7539b174aef4 ("x86: kvmguest: use TSC clocksource if
invariant TSC is exposed")).
Notable cloud vendors[1] and performance engineers[2] recommend that Xen
users preferentially select tsc over xen-clocksource due the performance
penalty incurred by the latter. These articles are persuasive and
tailored to specific use cases. In order to understand the tradeoffs
around this choice more fully, this author had to reference the
documented[3] complexities around the Xen configuration, as well as the
kernel's clocksource selection algorithm. Many users may not attempt
this to correctly configure the right clock source in their guest.
The approach taken in the kvm-clock module spares users this confusion,
where possible.
Both the Intel SDM[4] and the Xen tsc documentation explain that marking
a tsc as invariant means that it should be considered stable by the OS
and is elibile to be used as a wall clock source.
In order to obtain better out-of-the-box performance, and reduce the
need for user tuning, follow kvm's approach and decrease the xen clock
rating so that tsc is preferable, if it is invariant, stable, and the
tsc will never be emulated.
[1] https://aws.amazon.com/premiumsupport/knowledge-center/manage-ec2-linux-clock-source/
[2] https://www.brendangregg.com/blog/2021-09-26/the-speed-of-time.html
[3] https://xenbits.xen.org/docs/unstable/man/xen-tscmode.7.html
[4] Intel 64 and IA-32 Architectures Sofware Developer's Manual Volume
3b: System Programming Guide, Part 2, Section 17.17.1, Invariant TSC
Signed-off-by: Krister Johansen <kjlx@templeofstupid.com>
Code-reviewed-by: David Reaver <me@davidreaver.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20221216162118.GB2633@templeofstupid.com
Signed-off-by: Juergen Gross <jgross@suse.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.