From: Marcelo Tosatti <mtosatti@redhat.com>
To: Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@hitachi.com>
Cc: linux-kernel@vger.kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
kvm@vger.kernel.org, Joerg Roedel <joerg.roedel@amd.com>,
David Sharp <dhsharp@google.com>,
Steven Rostedt <rostedt@goodmis.org>,
Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>,
Ingo Molnar <mingo@redhat.com>, Avi Kivity <avi@redhat.com>,
yrl.pp-manager.tt@hitachi.com,
Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
Date: Fri, 16 Nov 2012 08:05:48 -0200 [thread overview]
Message-ID: <20121116100548.GA20892@amt.cnet> (raw)
In-Reply-To: <50A5F4C4.2030909@hitachi.com>
On Fri, Nov 16, 2012 at 05:09:40PM +0900, Yoshihiro YUNOMAE wrote:
> Hi Marcelo,
>
> Thank you for commenting on my patch set.
>
> (2012/11/16 12:19), Marcelo Tosatti wrote:
> >On Wed, Nov 14, 2012 at 10:36:21AM +0900, Yoshihiro YUNOMAE wrote:
> [...]
> >>In this summary, I suggest the patch which TSC offset for each guest can be
> >>output on the host.
> >
> >The guest TSC can change (for example if TSC scaling is used). Moreover
> >TSC offset can change, and you'd have to monitor that. What
>
> Yes, that's true. Changing TSC offset is the key point to use TSC for
> merging trace data of guests and the host.
>
> >about a module option so that tsc_offset is written as zero (to be
> >used as debugging tool). Then the following restrictions apply:
> >
> >- TSC must be synchronized across CPUs/VCPUS.
> >- TSC must be reliable.
> >
> >Would that suffice? (a module option to kvm.ko, say zero_tsc_offset).
>
> As you say, the guest TSC can change, so guest TSC needs to meet these
> two restrictions to merge the trace data in chronological order.
>
> However, the zero-TSC offset method is not enough, I think.
> I will use TSC values as the tracing timestamp not only for debugging
> but for failure analysis on actual operations. When we introduce
> the zero-TSC offset, normally it will be no problem. However, if
> the guest executes write_tsc or the guest works live migration, TSC
> offset will be changed. After all, we need to monitor the TSC offset
> value.
>
> Thank you,
What i wrote was not precise.
With TSC scaling (of AMD's svm.c) or TSC trapping, the guest RDTSC is
not monotonic with reference to the host TSC. That is, TSC scaling
and/or trapping are fundamentally incompatible with the feature you
propose.
What you are saying is that is you'd like this feature for production,
not debugging. Unfortunately this requires a synchronized TSC across
CPUs, which is not widespread. Therefore the suggestion to introduce a
debugging facility (you cannot rely on this for failure analysis on all
systems).
Even then, a more reliable method for reporting TSC offset must be used.
Apart from these issues, it is very useful to correlate guest/host
events in the way you propose.
prev parent reply other threads:[~2012-11-16 10:06 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-11-14 1:36 [RFC PATCH 0/2] kvm/vmx: Output TSC offset Yoshihiro YUNOMAE
2012-11-14 1:36 ` [RFC PATCH 1/2] kvm/vmx: Print TSC_OFFSET information when TSC offset value is written to VMCS Yoshihiro YUNOMAE
2012-11-14 1:37 ` [RFC PATCH 2/2] tools: Add a tool for merging trace data of a guest and a host Yoshihiro YUNOMAE
2012-11-14 2:00 ` [RFC PATCH 0/2] kvm/vmx: Output TSC offset Steven Rostedt
2012-11-14 2:02 ` H. Peter Anvin
2012-11-14 2:03 ` David Sharp
2012-11-14 2:31 ` Steven Rostedt
2012-11-14 8:26 ` Yoshihiro YUNOMAE
2012-11-16 15:05 ` Steven Rostedt
2012-11-16 18:56 ` Marcelo Tosatti
2012-11-20 10:38 ` Yoshihiro YUNOMAE
2012-11-16 19:15 ` Marcelo Tosatti
2012-11-20 10:36 ` Yoshihiro YUNOMAE
2012-11-20 22:51 ` Marcelo Tosatti
2012-11-22 5:21 ` Yoshihiro YUNOMAE
2012-11-23 22:46 ` Marcelo Tosatti
2012-11-26 11:05 ` Yoshihiro YUNOMAE
2012-11-26 23:16 ` Marcelo Tosatti
2012-11-27 10:53 ` Yoshihiro YUNOMAE
2012-11-29 22:51 ` Marcelo Tosatti
2012-11-30 1:36 ` Yoshihiro YUNOMAE
2012-11-30 20:42 ` Marcelo Tosatti
2012-12-03 0:55 ` Yoshihiro YUNOMAE
2012-11-16 3:19 ` Marcelo Tosatti
2012-11-16 8:09 ` Yoshihiro YUNOMAE
2012-11-16 10:05 ` Marcelo Tosatti [this message]
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