mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/
synced 2026-11-10 20:48:12 +00:00
No description
- C 96.8%
- Assembly 0.9%
- Rust 0.7%
- Shell 0.6%
- Python 0.5%
- Other 0.3%
a4248ee16f
When running in a virtual machine, we might see the original hardware CPU
vendor string (i.e. "AuthenticAMD"), but a model and family ID set by the
hypervisor. In case we run on AMD hardware and the hypervisor sets a model
ID < 0x14, the LAHF cpu feature is eliminated from the the list of CPU
capabilities present to circumvent a bug with some BIOSes in conjunction with
AMD K8 processors.
Parsing the flags list from /proc/cpuinfo seems to be happening mostly in
bash scripts and prebuilt Docker containers, as it does not need to have
additionals tools present – even though more reliable ways like using "kcpuid",
which calls the CPUID instruction instead of parsing a list, should be preferred.
Scripts, that use /proc/cpuinfo to determine if the current CPU is
"compliant" with defined microarchitecture levels like x86-64-v2 will falsely
claim the CPU is incapable of modern CPU instructions when "lahf_lm" is missing
in that flags list.
This can prevent some docker containers from starting or build scripts to create
unoptimized binaries.
Admittably, this is more a small inconvenience than a severe bug in the kernel
and the shoddy scripts that rely on parsing /proc/cpuinfo
should be fixed instead.
This patch adds an additional check to see if we're running inside a
virtual machine (X86_FEATURE_HYPERVISOR is present), which, to my
understanding, can't be present on a real K8 processor as it was introduced
only with the later/other Athlon64 models.
Example output with the "lahf_lm" flag missing in the flags list
(should be shown between "hypervisor" and "abm"):
$ cat /proc/cpuinfo
processor : 0
vendor_id : AuthenticAMD
cpu family : 15
model : 6
model name : Common KVM processor
stepping : 1
microcode : 0x1000065
cpu MHz : 2599.998
cache size : 512 KB
physical id : 0
siblings : 1
core id : 0
cpu cores : 1
apicid : 0
initial apicid : 0
fpu : yes
fpu_exception : yes
cpuid level : 13
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca
cmov pat pse36 clflush mmx fxsr sse sse2 syscall nx rdtscp
lm rep_good nopl cpuid extd_apicid tsc_known_freq pni
pclmulqdq ssse3 fma cx16 sse4_1 sse4_2 x2apic movbe popcnt
tsc_deadline_timer aes xsave avx f16c hypervisor abm
3dnowprefetch vmmcall bmi1 avx2 bmi2 xsaveopt
... while kcpuid shows the feature to be present in the CPU:
# kcpuid -d | grep lahf
lahf_lm - LAHF/SAHF available in 64-bit mode
[ mingo: Updated the comment a bit, incorporated Boris's review feedback. ]
Signed-off-by: Max Grobecker <max@grobecker.info>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: linux-kernel@vger.kernel.org
Cc: Borislav Petkov <bp@alien8.de>
|
History
|
|
|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| rust | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .clippy.toml | ||
| .cocciconfig | ||
| .editorconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| .rustfmt.toml | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| 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 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.