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547c91929f
When querying whether or not a vCPU "is" running in kernel mode, directly
get the CPL if the vCPU is the currently loaded vCPU. In scenarios where
a guest is profiled via perf-kvm, querying vcpu->arch.preempted_in_kernel
from kvm_guest_state() is wrong if vCPU is actively running, i.e. isn't
scheduled out due to being preempted and so preempted_in_kernel is stale.
This affects perf/core's ability to accurately tag guest RIP with
PERF_RECORD_MISC_GUEST_{KERNEL|USER} and record it in the sample. This
causes perf/tool to fail to connect the vCPU RIPs to the guest kernel
space symbols when parsing these samples due to incorrect PERF_RECORD_MISC
flags:
Before (perf-report of a cpu-cycles sample):
1.23% :58945 [unknown] [u] 0xffffffff818012e0
After:
1.35% :60703 [kernel.vmlinux] [g] asm_exc_page_fault
Note, checking preempted_in_kernel in kvm_arch_vcpu_in_kernel() is awful
as nothing in the API's suggests that it's safe to use if and only if the
vCPU was preempted. That can be cleaned up in the future, for now just
fix the glaring correctness bug.
Note #2, checking vcpu->preempted is NOT safe, as getting the CPL on VMX
requires VMREAD, i.e. is correct if and only if the vCPU is loaded. If
the target vCPU *was* preempted, then it can be scheduled back in after
the check on vcpu->preempted in kvm_vcpu_on_spin(), i.e. KVM could end up
trying to do VMREAD on a VMCS that isn't loaded on the current pCPU.
Signed-off-by: Like Xu <likexu@tencent.com>
Fixes:
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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.