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Date:   Fri, 16 Oct 2020 12:38:47 +0100
From:   Suzuki Poulose <>
To:     Leo Yan <>,
        Sai Prakash Ranjan <>
Cc:     Denis Nikitin <>,
        Mathieu Poirier <>,,,, Stephen Boyd <>,,
        Mike Leach <>
Subject: Re: [PATCH] coresight: etm4x: Add config to exclude kernel mode

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]

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 

>> 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 !)


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