linux-kernel - Re: [PATCH] kvm: tracing: Fix unmatched kvm_entry and kvm

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Message-ID: <59770b4e-0238-a1a4-83cb-77d36ff3b586@redhat.com>
Date:   Mon, 18 Jan 2021 19:20:00 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Dario Faggioli <dfaggioli@...e.com>, linux-kernel@...r.kernel.org
Cc:     Lorenzo Brescia <lorenzo.brescia@....unito.it>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org
Subject: Re: [PATCH] kvm: tracing: Fix unmatched kvm_entry and kvm_exit events

On 23/12/20 15:45, Dario Faggioli wrote:
> From: Lorenzo Brescia <lorenzo.brescia@....unito.it>
> 
> On VMX, if we exit and then re-enter immediately without leaving
> the vmx_vcpu_run() function, the kvm_entry event is not logged.
> That means we will see one (or more) kvm_exit, without its (their)
> corresponding kvm_entry, as shown here:
> 
>   CPU-1979 [002] 89.871187: kvm_entry: vcpu 1
>   CPU-1979 [002] 89.871218: kvm_exit:  reason MSR_WRITE
>   CPU-1979 [002] 89.871259: kvm_exit:  reason MSR_WRITE
> 
> It also seems possible for a kvm_entry event to be logged, but then
> we leave vmx_vcpu_run() right away (if vmx->emulation_required is
> true). In this case, we will have a spurious kvm_entry event in the
> trace.
> 
> Fix these situations by moving trace_kvm_entry() inside vmx_vcpu_run()
> (where trace_kvm_exit() already is).
> 
> A trace obtained with this patch applied looks like this:
> 
>   CPU-14295 [000] 8388.395387: kvm_entry: vcpu 0
>   CPU-14295 [000] 8388.395392: kvm_exit:  reason MSR_WRITE
>   CPU-14295 [000] 8388.395393: kvm_entry: vcpu 0
>   CPU-14295 [000] 8388.395503: kvm_exit:  reason EXTERNAL_INTERRUPT
> 
> Of course, not calling trace_kvm_entry() in common x86 code any
> longer means that we need to adjust the SVM side of things too.
> 
> Signed-off-by: Lorenzo Brescia <lorenzo.brescia@....unito.it>
> Signed-off-by: Dario Faggioli <dfaggioli@...e.com>
> ---
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Sean Christopherson <seanjc@...gle.com>
> Cc: Vitaly Kuznetsov <vkuznets@...hat.com>
> Cc: Wanpeng Li <wanpengli@...cent.com>
> Cc: Jim Mattson <jmattson@...gle.com>
> Cc: Joerg Roedel <joro@...tes.org>
> Cc: kvm@...r.kernel.org
> Cc: Lorenzo Brescia <lorenzo.brescia@....unito.it>
> Cc: Dario Faggioli <dfaggioli@...e.com>
> ---
>   arch/x86/kvm/svm/svm.c |    2 ++
>   arch/x86/kvm/vmx/vmx.c |    2 ++
>   arch/x86/kvm/x86.c     |    3 +--
>   3 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index cce0143a6f80..ed272fcf6495 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3741,6 +3741,8 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu)
>   {
>   	struct vcpu_svm *svm = to_svm(vcpu);
>   
> +	trace_kvm_entry(vcpu);
> +
>   	svm->vmcb->save.rax = vcpu->arch.regs[VCPU_REGS_RAX];
>   	svm->vmcb->save.rsp = vcpu->arch.regs[VCPU_REGS_RSP];
>   	svm->vmcb->save.rip = vcpu->arch.regs[VCPU_REGS_RIP];
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 75c9c6a0a3a4..ff20f9e6e5b3 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6653,6 +6653,8 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
>   	if (vmx->emulation_required)
>   		return EXIT_FASTPATH_NONE;
>   
> +	trace_kvm_entry(vcpu);
> +
>   	if (vmx->ple_window_dirty) {
>   		vmx->ple_window_dirty = false;
>   		vmcs_write32(PLE_WINDOW, vmx->ple_window);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 3f7c1fc7a3ce..a79666204907 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8973,8 +8973,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>   		kvm_x86_ops.request_immediate_exit(vcpu);
>   	}
>   
> -	trace_kvm_entry(vcpu);
> -
>   	fpregs_assert_state_consistent();
>   	if (test_thread_flag(TIF_NEED_FPU_LOAD))
>   		switch_fpu_return();
> @@ -11538,6 +11536,7 @@ int kvm_sev_es_string_io(struct kvm_vcpu *vcpu, unsigned int size,
>   }
>   EXPORT_SYMBOL_GPL(kvm_sev_es_string_io);
>   
> +EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_entry);
>   EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_exit);
>   EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_fast_mmio);
>   EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_inj_virq);
> 
> 

Qeuued, thanks!

Paolo