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

[Marcelo Tosatti <mtosatti@redhat.com>]

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).