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KVM: x86: Get CPL directly when checking if loaded vCPU is in kernel mode

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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: e1bfc24577 ("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: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Like Xu 2023-11-23 15:58:18 +08:00 committed by Sean Christopherson
parent ffc253263a
commit 547c91929f
1 changed files with 4 additions and 1 deletions
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5
arch/x86/kvm/x86.c
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@ -12910,7 +12910,10 @@ bool kvm_arch_vcpu_in_kernel(struct kvm_vcpu *vcpu)
if (vcpu->arch.guest_state_protected)
return true;
return vcpu->arch.preempted_in_kernel;
if (vcpu != kvm_get_running_vcpu())
return vcpu->arch.preempted_in_kernel;
return static_call(kvm_x86_get_cpl)(vcpu) == 0;
}
unsigned long kvm_arch_vcpu_get_ip(struct kvm_vcpu *vcpu)