linux-kernel - Re: [PATCH 2/2] KVM: x86/pmu: Protect kvm->arch.pmu_event

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Date:   Fri, 18 Feb 2022 18:25:39 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Like Xu <like.xu.linux@...il.com>,
        Eric Hankland <ehankland@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: x86/pmu: Protect kvm->arch.pmu_event_filter
 with SRCU

On Fri, Feb 18, 2022, Paolo Bonzini wrote:
> On 2/17/22 09:36, Like Xu wrote:
> > From: Like Xu<likexu@...cent.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.

Agreed, I bet similar warnings can be triggered on SVM with nrips=false due to
svm_skip_emulated_instruction() dropping into the emulator, e.g. for HyperV and
Xen usage where next_rip doesn't appear to be filled in all paths.