Re: [PATCH] coresight: etm4x: Add config to exclude kernel mode tracing

[Suzuki Poulose]

On 10/16/20 10:24 AM, Leo Yan wrote:
> Hi Sai,
>
> On Fri, Oct 16, 2020 at 02:10:47PM +0530, Sai Prakash Ranjan wrote:
> > Hi Leo,
> >
> > On 2020-10-16 12:54, Leo Yan wrote:
> > > On Thu, Oct 15, 2020 at 11:40:05PM -0700, Denis Nikitin wrote:
> > > > Hi Mathieu,
> > > >
> > > > I think one of the use cases could be VMs.
> > > > Is there isolation between EL1 guest kernels which we can control
> > > > from perf
> > > > in a system wide mode?
> > >
> > > Sorry for suddenly jumping in.
> > >
> > > For KVM, I think we need to implement mechanism for saving/restoring
> > > CoreSight context for every guest OS, the CPU PMUs has implemented
> > > related features [1].
> > >
> > > Thanks,
> > > Leo
> > >
> > > [1]
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/kvm/pmu.c

Its not as easy as the CPU PMU for virtualizing the ETMs (with memory
mapped access only), i.e supporting ETMs from VMs.
We could definitely stop/resume on guest entry/exit, to support 
attr.exclude_guest.

> > What happens to the sysfs mode of tracing? For that we would still
> > need a config right to exclude kernel mode tracing completely.
>
> IIUC, sysfs mode and perf mode both can apply the same approach, the
> guest OS runs a thread context for the host, so when a guest OS is
> switched in or out, the hypervisor can save/restore the context for
> the guest OS; thus every guest OS will have its dedicated context and
> trace data ideally.

I don't think Guest Context is something we can support as mentioned
above, at least for systems without sysreg access for ETMs (and 
virtualizing ETRs is a different story !)

Cheers
Suzuki