linux-kernel - Re: [PATCH 6/7] KVM: x86: Use common definition for kvm_nested

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Date:   Tue, 21 Jul 2020 17:42:33 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 6/7] KVM: x86: Use common definition for
 kvm_nested_vmexit tracepoint

On Tue, 21 Jul 2020 12:31:30 -0700
Sean Christopherson <sean.j.christopherson@...el.com> wrote:

> +Steve
> 
> Background: KVM has two tracepoints that effectively trace the same thing
> (VM-Exit vs. nested VM-Exit), but use completely different formatting and
> nomenclature for each of the existing tracepoints.  I want to add a common
> macro to create the tracepoints so that they capture the exact same info
> and report it with the exact same format.  But that means breaking the
> "ABI" for one of the tracepoints, e.g. trace-cmd barfs on the rename of
> exit_code to exit_reason.

Feel free to update it.

> 
> Was there ever a verdict on whether or not tracepoints are considered ABI
> and thus must retain backwards compatibility?
> 
> If not, what's the proper way to upstream changes to trace-cmd?
> 

There's a kvm plugin in the libtraceevent code.

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/lib/traceevent/plugins/plugin_kvm.c

This overrides how the events are read. In the callback handler
(e.g. kvm_nested_vmexit_handle()), you can test if "rip" is there or
not. If it is not, you can do something different. For example:

	struct tep_format_field *field;

	field = tep_find_any_field(event, "rip");
	if (field) {
		tep_print_num_field(s, "rip %llx ", event, "rip", record, 1)
		[..]
	} else {
		/* do something new */
	}

You can test if fields exist and have the plugins do different things
depending on the format of an event. This is what I do in case an event
changes in the future.

-- Steve