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

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Message-ID: <f6ee4156-664c-7bff-8e35-5cd1acdbaa84@arm.com>
Date:   Fri, 16 Oct 2020 12:38:47 +0100
From:   Suzuki Poulose <suzuki.poulose@....com>
To:     Leo Yan <leo.yan@...aro.org>,
        Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
Cc:     Denis Nikitin <denik@...omium.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        linux-arm-msm@...r.kernel.org, coresight@...ts.linaro.org,
        linux-kernel@...r.kernel.org, Stephen Boyd <swboyd@...omium.org>,
        linux-arm-kernel@...ts.infradead.org,
        Mike Leach <mike.leach@...aro.org>
Subject: Re: [PATCH] coresight: etm4x: Add config to exclude kernel mode
 tracing

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