1. 29 Nov, 2023 1 commit
    • Like Xu's avatar
      KVM: x86: Get CPL directly when checking if loaded vCPU is in kernel mode · 547c9192
      Like Xu authored
      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: default avatarLike Xu <likexu@tencent.com>
      Fixes: e1bfc245 ("KVM: Move x86's perf guest info callbacks to generic KVM")
      Link: https://lore.kernel.org/r/20231123075818.12521-1-likexu@tencent.com
      [sean: massage changelong, add Fixes]
      Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
      547c9192
  2. 30 Oct, 2023 1 commit
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