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a7cec20845
Allow a guest to read the physical IA32_APERF and IA32_MPERF MSRs without interception. The IA32_APERF and IA32_MPERF MSRs are not virtualized. Writes are not handled at all. The MSR values are not zeroed on vCPU creation, saved on suspend, or restored on resume. No accommodation is made for processor migration or for sharing a logical processor with other tasks. No adjustments are made for non-unit TSC multipliers. The MSRs do not account for time the same way as the comparable PMU events, whether the PMU is virtualized by the traditional emulation method or the new mediated pass-through approach. Nonetheless, in a properly constrained environment, this capability can be combined with a guest CPUID table that advertises support for CPUID.6:ECX.APERFMPERF[bit 0] to induce a Linux guest to report the effective physical CPU frequency in /proc/cpuinfo. Moreover, there is no performance cost for this capability. Signed-off-by: Jim Mattson <jmattson@google.com> Link: https://lore.kernel.org/r/20250530185239.2335185-3-jmattson@google.com Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com> Link: https://lore.kernel.org/r/20250626001225.744268-3-seanjc@google.com Signed-off-by: Sean Christopherson <seanjc@google.com> |
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| drm | ||
| linux | ||
| README | ||
Why we want a copy of kernel headers in tools?
==============================================
There used to be no copies, with tools/ code using kernel headers
directly. From time to time tools/perf/ broke due to legitimate kernel
hacking. At some point Linus complained about such direct usage. Then we
adopted the current model.
The way these headers are used in perf are not restricted to just
including them to compile something.
There are sometimes used in scripts that convert defines into string
tables, etc, so some change may break one of these scripts, or new MSRs
may use some different #define pattern, etc.
E.g.:
$ ls -1 tools/perf/trace/beauty/*.sh | head -5
tools/perf/trace/beauty/arch_errno_names.sh
tools/perf/trace/beauty/drm_ioctl.sh
tools/perf/trace/beauty/fadvise.sh
tools/perf/trace/beauty/fsconfig.sh
tools/perf/trace/beauty/fsmount.sh
$
$ tools/perf/trace/beauty/fadvise.sh
static const char *fadvise_advices[] = {
[0] = "NORMAL",
[1] = "RANDOM",
[2] = "SEQUENTIAL",
[3] = "WILLNEED",
[4] = "DONTNEED",
[5] = "NOREUSE",
};
$
The tools/perf/check-headers.sh script, part of the tools/ build
process, points out changes in the original files.
So its important not to touch the copies in tools/ when doing changes in
the original kernel headers, that will be done later, when
check-headers.sh inform about the change to the perf tools hackers.
Another explanation from Ingo Molnar:
It's better than all the alternatives we tried so far:
- Symbolic links and direct #includes: this was the original approach but
was pushed back on from the kernel side, when tooling modified the
headers and broke them accidentally for kernel builds.
- Duplicate self-defined ABI headers like glibc: double the maintenance
burden, double the chance for mistakes, plus there's no tech-driven
notification mechanism to look at new kernel side changes.
What we are doing now is a third option:
- A software-enforced copy-on-write mechanism of kernel headers to
tooling, driven by non-fatal warnings on the tooling side build when
kernel headers get modified:
Warning: Kernel ABI header differences:
diff -u tools/include/uapi/drm/i915_drm.h include/uapi/drm/i915_drm.h
diff -u tools/include/uapi/linux/fs.h include/uapi/linux/fs.h
diff -u tools/include/uapi/linux/kvm.h include/uapi/linux/kvm.h
...
The tooling policy is to always pick up the kernel side headers as-is,
and integate them into the tooling build. The warnings above serve as a
notification to tooling maintainers that there's changes on the kernel
side.
We've been using this for many years now, and it might seem hacky, but
works surprisingly well.