linux-kernel - Re: [PATCH v2 0/4] KVM: x86: tracepoint updates

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Date:   Tue, 26 Sep 2023 11:28:35 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, Dave Hansen <dave.hansen@...ux.intel.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v2 0/4] KVM: x86: tracepoint updates

У пн, 2023-09-25 у 17:03 -0700, Sean Christopherson пише:
> On Sun, Sep 24, 2023, Maxim Levitsky wrote:
> > This patch series is intended to add some selected information
> > to the kvm tracepoints to make it easier to gather insights about
> > running nested guests.
> > 
> > This patch series was developed together with a new x86 performance analysis tool
> > that I developed recently (https://gitlab.com/maximlevitsky/kvmon)
> > which aims to be a better kvm_stat, and allows you at glance
> > to see what is happening in a VM, including nesting.
> 
> Rather than add more and more tracepoints, I think we should be more thoughtful
> about (a) where we place KVM's tracepoints and (b) giving userspace the necessary
> hooks to write BPF programs to extract whatever data is needed at any give time.
> 
> There's simply no way we can iterate fast enough in KVM tracepoints to adapt to
> userspace's debug/monitoring needs.  E.g. if it turns out someone wants detailed
> info on hypercalls that use memory or registers beyond ABCD, the new tracepoints
> won't help them.
> 
> If all KVM tracepoints grab "struct kvm_vcpu" and force VMCS "registers" to be
> cached (or decached depending on one's viewpoint), then I think that'll serve 99%
> of usecases.  E.g. the vCPU gives a BPF program kvm_vcpu, vcpu_{vmx,svm}, kvm, etc.
> 
> trace_kvm_exit is good example, where despite all of the information that is captured
> by KVM, it's borderline worthless for CPUID and MSR exits because their interesting
> information is held in registers and not captured in the VMCS or VMCB.
> 
> There are some on BTF type info issues that I've encountered, but I suspect that's
> as much a PEBKAC problem as anything.
> 

While eBPF has its use cases, none of the extra tracepoints were added solely because of
the monitoring tool and I do understand that tracepoints are a limited resource.

Each added tracepoint/info was added only when it was also found to be useful for regular
kvm tracing.

Best regards,
	Maxim Levitsky