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Krister Johansen caea091e48 x86/xen/time: prefer tsc as clocksource when it is invariant 
Kvm elects to use tsc instead of kvm-clock when it can detect that the
TSC is invariant.

(As of commit 7539b174ae ("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>
2023-02-13 06:53:20 +01:00
arch x86/xen/time: prefer tsc as clocksource when it is invariant 2023-02-13 06:53:20 +01:00
block block-6.2-2023-02-03 2023-02-03 11:35:42 -08:00
certs certs: Fix build error when PKCS#11 URI contains semicolon 2023-01-31 17:53:01 +09:00
crypto This update includes the following changes: 2022-12-14 12:31:09 -08:00
Documentation drivers/xen/hypervisor: Expose Xen SIF flags to userspace 2023-02-13 06:53:19 +01:00
drivers drivers/xen/hypervisor: Expose Xen SIF flags to userspace 2023-02-13 06:53:19 +01:00
fs for-6.2-rc7-tag 2023-02-12 11:26:36 -08:00
include tracing: Fix showing of TASK_COMM_LEN instead of its value 2023-02-12 13:52:17 -08:00
init Kbuild fixes for v6.2 (3rd) 2023-01-21 10:56:37 -08:00
io_uring io_uring: always prep_async for drain requests 2023-01-27 06:29:29 -07:00
ipc Non-MM patches for 6.2-rc1. 2022-12-12 17:28:58 -08:00
kernel tracing: Fix showing of TASK_COMM_LEN instead of its value 2023-02-12 13:52:17 -08:00
lib - Lock the proper critical section when dealing with perf event context 2023-02-05 11:03:56 -08:00
LICENSES LICENSES: Add the copyleft-next-0.3.1 license 2022-11-08 15:44:01 +01:00
mm Fix page corruption caused by racy check in __free_pages 2023-02-12 10:30:05 -08:00
net rds: rds_rm_zerocopy_callback() use list_first_entry() 2023-02-09 10:37:26 +01:00
rust rust: print: avoid evaluating arguments in pr_* macros in unsafe blocks 2023-01-16 00:54:35 +01:00
samples ftrace: Export ftrace_free_filter() to modules 2023-01-24 11:20:58 -05:00
scripts Kbuild fixes for v6.2 (4th) 2023-02-04 11:30:23 -08:00
security tomoyo: Update website link 2023-01-13 23:11:38 +09:00
sound sound fixes for 6.2-rc8 2023-02-10 08:37:48 -08:00
tools x86/xen: mark xen_pv_play_dead() as __noreturn 2023-02-13 06:53:19 +01:00
usr usr/gen_init_cpio.c: remove unnecessary -1 values from int file 2022-10-03 14:21:44 -07:00
virt VFIO fixes for v6.2-rc6 2023-01-23 11:56:07 -08:00
.clang-format iommufd for 6.2 2022-12-14 09:15:43 -08:00
.cocciconfig
.get_maintainer.ignore get_maintainer: add Alan to .get_maintainer.ignore 2022-08-20 15:17:44 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore .gitignore: ignore *.rpm 2022-12-30 17:22:14 +09:00
.mailmap 25 hotfixes, mainly for MM. 13 are cc:stable. 2023-02-03 10:01:57 -08:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: update SCTP maintainers 2023-02-02 11:35:33 -08:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS MAINTAINERS: Add myself as maintainer for arch/sh (SUPERH) 2023-02-12 13:57:09 -08:00
Makefile Linux 6.2-rc8 2023-02-12 14:10:17 -08:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

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.