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

[Paolo Bonzini]

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'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.

Paolo