linux-kernel - Re: [PATCH 4/4] kvm: x86: export TSC offset to user-space

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Message-ID: <20160902101541.2b670b09@gandalf.local.home>
Date:   Fri, 2 Sep 2016 10:15:41 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Stefan Hajnoczi <stefanha@...il.com>
Cc:     Luiz Capitulino <lcapitulino@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, pbonzini@...hat.com,
        rkrcmar@...hat.com, mhiramat@...nel.org, mtosatti@...hat.com
Subject: Re: [PATCH 4/4] kvm: x86: export TSC offset to user-space

On Fri, 2 Sep 2016 09:43:01 -0400
Stefan Hajnoczi <stefanha@...il.com> wrote:

> Can TSC offset changes occur at runtime?
> 
> One example is vcpu hotplug where the tracing tool would need to fetch
> the new vcpu's TSC offset after tracing has already started.
> 
> Another example is if QEMU or the guest change the TSC offset while
> running.  If the tracing tool doesn't notice this then trace events will have
> incorrect timestamps.

I believe there are tracepoints for these events. They would obviously
need to be enabled for the tracer to catch them.

I would also recommend that they go into their own instance to make
sure other events do not overwrite them.

-- Steve