linux-kernel - Re: [PATCH v5 3/3] KVM: x86: add new nested vmexit tracepoints

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Message-ID: <9ff2be87-117a-4f96-af3b-dacb55467449@redhat.com>
Date: Thu, 19 Dec 2024 18:33:00 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc: x86@...nel.org, Dave Hansen <dave.hansen@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>,
 Ingo Molnar <mingo@...hat.com>, Sean Christopherson <seanjc@...gle.com>,
 "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 3/3] KVM: x86: add new nested vmexit tracepoints

On 9/10/24 22:03, Maxim Levitsky wrote:
> Add 3 new tracepoints for nested VM exits which are intended
> to capture extra information to gain insights about the nested guest
> behavior.
> 
> The new tracepoints are:
> 
> - kvm_nested_msr
> - kvm_nested_hypercall
> 
> These tracepoints capture extra register state to be able to know
> which MSR or which hypercall was done.
> 
> - kvm_nested_page_fault
> 
> This tracepoint allows to capture extra info about which host pagefault
> error code caused the nested page fault.
> 
> Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
> ---
>   arch/x86/kvm/svm/nested.c | 22 +++++++++++
>   arch/x86/kvm/trace.h      | 82 +++++++++++++++++++++++++++++++++++++--
>   arch/x86/kvm/vmx/nested.c | 27 +++++++++++++
>   arch/x86/kvm/x86.c        |  3 ++
>   4 files changed, 131 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index 6f704c1037e51..2020307481553 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -38,6 +38,8 @@ static void nested_svm_inject_npf_exit(struct kvm_vcpu *vcpu,
>   {
>   	struct vcpu_svm *svm = to_svm(vcpu);
>   	struct vmcb *vmcb = svm->vmcb;
> +	u64 host_error_code = vmcb->control.exit_info_1;
> +
>   
>   	if (vmcb->control.exit_code != SVM_EXIT_NPF) {
>   		/*
> @@ -48,11 +50,15 @@ static void nested_svm_inject_npf_exit(struct kvm_vcpu *vcpu,
>   		vmcb->control.exit_code_hi = 0;
>   		vmcb->control.exit_info_1 = (1ULL << 32);
>   		vmcb->control.exit_info_2 = fault->address;
> +		host_error_code = 0;
>   	}
>   
>   	vmcb->control.exit_info_1 &= ~0xffffffffULL;
>   	vmcb->control.exit_info_1 |= fault->error_code;
>   
> +	trace_kvm_nested_page_fault(fault->address, host_error_code,
> +				    fault->error_code);
> +

I disagree with Sean about trace_kvm_nested_page_fault.  It's a useful 
addition and it is easier to understand what's happening with a 
dedicated tracepoint (especially on VMX).

Tracepoint are not an exact science and they aren't entirely kernel API. 
  At least they can just go away at any time (changing them is a lot 
more tricky, but their presence is not guaranteed).  The one below has 
the slight ugliness of having to do some computation in 
nested_svm_vmexit(), this one should go in.

>   	nested_svm_vmexit(svm);
>   }
>   
> @@ -1126,6 +1132,22 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
>   				       vmcb12->control.exit_int_info_err,
>   				       KVM_ISA_SVM);
>   
> +	/* Collect some info about nested VM exits */
> +	switch (vmcb12->control.exit_code) {
> +	case SVM_EXIT_MSR:
> +		trace_kvm_nested_msr(vmcb12->control.exit_info_1 == 1,
> +				     kvm_rcx_read(vcpu),
> +				     (vmcb12->save.rax & 0xFFFFFFFFull) |
> +				     (((u64)kvm_rdx_read(vcpu) << 32)));
> +		break;
> +	case SVM_EXIT_VMMCALL:
> +		trace_kvm_nested_hypercall(vmcb12->save.rax,
> +					   kvm_rbx_read(vcpu),
> +					   kvm_rcx_read(vcpu),
> +					   kvm_rdx_read(vcpu));
> +		break;

Here I probably would have preferred an unconditional tracepoint giving 
RAX/RBX/RCX/RDX after a nested vmexit.  This is not exactly what Sean 
wanted but perhaps it strikes a middle ground?  I know you wrote this 
for a debugging tool, do you really need to have everything in a single 
tracepoint, or can you correlate the existing exit tracepoint with this 
hypothetical trace_kvm_nested_exit_regs, to pick RDMSR vs. WRMSR?

Paolo