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

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Message-ID: <Z4mN2Skhp1lQwrYw@google.com>
Date: Thu, 16 Jan 2025 14:53:13 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org, 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>, "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 Thu, Dec 19, 2024, Paolo Bonzini wrote:
> On 12/19/24 18:49, Maxim Levitsky wrote:
> > > 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?
> > 
> > Hi!
> > 
> > If the new trace_kvm_nested_exit_regs tracepoint has a VM exit number
> > argument, then I can enable this new tracepoint twice with a different
> > filter (vm_exit_num number == msr and vm_exit_num == vmcall), and each
> > instance will count the events that I need.
> > 
> > So this can work.
> Ok, thanks.  On one hand it may make sense to have trace_kvm_exit_regs and
> trace_kvm_nested_exit_regs (you can even extend the TRACE_EVENT_KVM_EXIT
> macro to generate both the exit and the exit_regs tracepoint).  On the other
> hand it seems to me that this new tracepoint is kinda reinventing the wheel;
> your patch adding nested equivalents of trace_kvm_hypercall and
> trace_kvm_msr seems more obvious to me.
> 
> I see Sean's point in not wanting one-off tracepoints, on the other hand
> there is value in having similar tracepoints for the L1->L0 and L2->L0
> cases.

I don't understand why we want two (or three, or five) tracepoints for the same
thing.  I want to go the opposite direction and (a) delete kvm_nested_vmexit
and then (b) rename kvm_nested_vmexit_inject => kvm_nested_vmexit so that it
pairs with kvm_nested_vmenter.

Similary, having kvm_nested_intr_vmexit is asinine when kvm_nested_vmexit_inject
captures *more* information about the IRQ itself.

I don't see the point of trace_kvm_nested_exit_regs.  Except for L1 vs. L2, it's
redundant.   kvm_nested_vmexit_inject and kvm_nested_vmenter are useful because
they capture novel information.

> I'll let him choose between the two possibilities (a new *_exit_regs
> pair, or just apply this patch) but I think there should be one of these
> two.

Anything but a pair.  Why can't we capture L1 vs. L2 in the tracepoints and call
it a day?