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SubjectRe: [PATCH 2/2] KVM: x86/pmu: Protect kvm->arch.pmu_event_filter with SRCU
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On 2/17/22 09:36, Like Xu wrote:
> From: Like Xu<likexu@tencent.com>
>
> Fix the following positive warning:
>
> =============================
> WARNING: suspicious RCU usage
> arch/x86/kvm/pmu.c:190 suspicious rcu_dereference_check() usage!
> other info that might help us debug this:
> rcu_scheduler_active = 2, debug_locks = 1
> 1 lock held by CPU 28/KVM/370841:
> #0: ff11004089f280b8 (&vcpu->mutex){+.+.}-{3:3}, at: kvm_vcpu_ioctl+0x87/0x730 [kvm]
> Call Trace:
> <TASK>
> dump_stack_lvl+0x59/0x73
> reprogram_fixed_counter+0x15d/0x1a0 [kvm]
> kvm_pmu_trigger_event+0x1a3/0x260 [kvm]
> ? free_moved_vector+0x1b4/0x1e0
> complete_fast_pio_in+0x8a/0xd0 [kvm]
> [...]

I think the right fix is to add SRCU protection to complete_userspace_io
in kvm_arch_vcpu_ioctl_run. Most calls of complete_userspace_io can
execute similar code to vmexits.

> Fixes: 66bb8a065f5a ("KVM: x86: PMU Event Filter")

It fixes 9cd803d496e7 ("KVM: x86: Update vPMCs when retiring
instructions", 2022-01-07), actually. That is when the PMU filter was
added to kvm_skip_emulated_instruction (called by kvm_fast_pio_in).

Thanks,

Paolo

> It's possible to call KVM_SET_PMU_EVENT_FILTER ioctl with the vCPU running.
> Similar to "kvm->arch.msr_filter", KVM should guarantee that vCPUs will
> see either the previous filter or the new filter so that guest pmu events
> with identical settings in both the old and new filter have deterministic
> behavior.

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