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Date: Fri, 25 Mar 2016 10:22:03 +0800
From: Peter Xu <peterx@redhat.com>
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Subject: Re: [Qemu-devel] [RFC] host and guest kernel trace merging
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To: Stefan Hajnoczi <stefanha@redhat.com>
Cc: kvm@vger.kernel.org, Stefan Hajnoczi <stefanha@gmail.com>, yoshihiro.yunomae.ez@hitachi.com, mtosatti@redhat.com, qemu-devel@nongnu.org, rostedt@goodmis.org, Luiz Capitulino <lcapitulino@redhat.com>, linux-trace-users@vger.kernel.org, pbonzini@redhat.com

On Thu, Mar 24, 2016 at 10:13:17AM +0000, Stefan Hajnoczi wrote:
> There are probably race conditions if the tsc offset is queried
> independently from the trace collection.  For example, imagine the host
> is suspend right when tracing begins.  I think the TSC could be adjusted
> when the host wakes up again.

Right... So maybe we should never allow tsc-offset change
(e.g. suspend) happen during host-guest tracing?  It seems more like
a question about "whether we can do a merge", rather than "read a
correct offset"... If it changes, we cannot do the merge any more
(only if we record the offset for each guest entry)...

> 
> Ideally the TSC information would be part of the trace data so that
> there are no race conditions when interpeting time stamps.

Agree. We should keep the tsc-offset in the trace data.

Thanks.

-- peterx