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

[Leo Yan]

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
> > 
> 
> 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.

Thanks,
Leo