From: Luiz Capitulino <lcapitulino@redhat.com>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Stefan Hajnoczi <stefanha@gmail.com>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
pbonzini@redhat.com, rkrcmar@redhat.com, rostedt@goodmis.org,
mhiramat@kernel.org
Subject: Re: [PATCH 4/4] kvm: x86: export TSC offset to user-space
Date: Fri, 2 Sep 2016 21:29:13 -0400 [thread overview]
Message-ID: <20160902212913.3d38a337@redhat.com> (raw)
In-Reply-To: <20160902234936.GA12659@amt.cnet>
On Fri, 2 Sep 2016 20:49:37 -0300
Marcelo Tosatti <mtosatti@redhat.com> wrote:
> On Fri, Sep 02, 2016 at 09:43:01AM -0400, Stefan Hajnoczi wrote:
> > On Wed, Aug 31, 2016 at 01:05:45PM -0400, Luiz Capitulino wrote:
> > > We need to retrieve a VM's TSC offset in order to use
> > > the host's TSC to merge host and guest traces. This is
> > > explained in detail in this thread:
> > >
> > > [Qemu-devel] [RFC] host and guest kernel trace merging
> > > https://lists.nongnu.org/archive/html/qemu-devel/2016-03/msg00887.html
> > >
> > > Today, the only way to retrieve a VM's TSC offset is
> > > by using the kvm_write_tsc_offset tracepoint. This has
> > > a few problems. First, the tracepoint is only emitted
> > > when the VM boots, which requires a reboot to get it if
> > > the VM is already running. Second, tracepoints are not
> > > supposed to be ABIs in case they need to be consumed by
> > > user-space tools.
> > >
> > > This commit exports a VM's TSC offset to user-space via
> > > debugfs. A new file called "tsc-offset" is created in
> > > the VM's debugfs directory. For example:
> > >
> > > /sys/kernel/debug/kvm/51696-10/tsc-offset
> > >
> > > This file contains one TSC offset per line, for each
> > > vCPU. For example:
> > >
> > > vcpu0: 18446742405270834952
> > > vcpu1: 18446742405270834952
> > > vcpu2: 18446742405270834952
> > > vcpu3: 18446742405270834952
> > >
> > > There are some important observations about this
> > > solution:
> > >
> > > - While all vCPUs TSC offsets should be equal for the
> > > cases we care about (ie. stable TSC and no write to
> > > the TSC MSR), I chose to follow the spec and export
> > > each vCPU's TSC offset (might also be helpful for
> > > debugging)
> > >
> > > - The TSC offset is only useful after the VM has booted
> > >
> > > - We'll probably need to export the TSC multiplier too.
> > > However, I've been using only the TSC offset for now.
> > > So, let's get this merged first and do the TSC multiplier
> > > as a second step
> >
> > 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.
> >
> > Stefan
>
> Yes they can, and the interface should mention that "the user is
> responsible for handling races of execution" (IMO).
>
> So the workflow is:
>
> 1) User boots VM and knows the state of the VM.
> 2) User runs trace-cmd on the host.
>
> Is there a need to automate gathering of traces? (that is to know the
> state of reboots and so forth). I don't see one. In that case, the above
> workflow is functional.
>
> Can you add such comments to the interface Luiz (that the value
> read is potentially stale).
Sure, no problem.
next prev parent reply other threads:[~2016-09-03 1:29 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-08-31 17:05 [PATCH 0/4] kvm: export TSC offset to user-space Luiz Capitulino
2016-08-31 17:05 ` [PATCH 1/4] kvm: kvm_destroy_vm_debugfs(): check debugs_stat_data pointer Luiz Capitulino
2016-09-02 13:51 ` Paolo Bonzini
2016-08-31 17:05 ` [PATCH 2/4] kvm: kvm_create_vm_debugfs(): cleanup on error Luiz Capitulino
2016-09-02 13:53 ` Paolo Bonzini
2016-08-31 17:05 ` [PATCH 3/4] kvm: add stub for arch specific debugfs support Luiz Capitulino
2016-09-02 13:53 ` Paolo Bonzini
2016-09-03 3:34 ` Masami Hiramatsu
2016-08-31 17:05 ` [PATCH 4/4] kvm: x86: export TSC offset to user-space Luiz Capitulino
2016-09-02 13:43 ` Stefan Hajnoczi
2016-09-02 14:15 ` Steven Rostedt
2016-09-03 0:23 ` Marcelo Tosatti
2016-09-03 4:04 ` Masami Hiramatsu
2016-09-02 16:26 ` Luiz Capitulino
2016-09-02 16:29 ` Luiz Capitulino
2016-09-02 23:49 ` Marcelo Tosatti
2016-09-03 1:29 ` Luiz Capitulino [this message]
2016-09-02 17:00 ` Paolo Bonzini
2016-09-02 17:31 ` Luiz Capitulino
2016-09-05 8:10 ` Paolo Bonzini
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20160902212913.3d38a337@redhat.com \
--to=lcapitulino@redhat.com \
--cc=kvm@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mhiramat@kernel.org \
--cc=mtosatti@redhat.com \
--cc=pbonzini@redhat.com \
--cc=rkrcmar@redhat.com \
--cc=rostedt@goodmis.org \
--cc=stefanha@gmail.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).