linux-kernel - Re: [RFC PATCH 0/3] coresight: Support exclude_guest with Feat

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Date:   Sat, 5 Aug 2023 11:28:39 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Marc Zyngier <maz@...nel.org>, James Clark <james.clark@....com>
Cc:     coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.linux.dev, Oliver Upton <oliver.upton@...ux.dev>,
        James Morse <james.morse@....com>,
        Zenghui Yu <yuzenghui@...wei.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mike Leach <mike.leach@...aro.org>,
        Leo Yan <leo.yan@...aro.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Rob Herring <robh@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] coresight: Support exclude_guest with Feat_TRF
 and nVHE

Hi Marc

On 04/08/2023 20:09, Marc Zyngier wrote:
> On Fri, 04 Aug 2023 11:13:10 +0100,
> James Clark <james.clark@....com> wrote:
>>
>> Hi,
>>
>> I'm looking for help in testing this and for feedback on whether it's
>> useful to anyone. Testing it requires hardware that has Feat_TRF (v8.4)
>> but no TRBE. This is because TRBE usage is disabled in nVHE guests.
>>
>> I don't currently have any access to any hardware, and the FVP model
>> can only do self hosted trace using TRBE.
>>
>> Currently with nVHE you would always get trace from guests, and
>> filtering out isn't possible without this patchset. In comparison, with
>> VHE guests, they never generate guest trace without [1]. I think the
>> existence of trace rather than lack of could suggest that this change is
>> less useful than [1]. Also the restricted set of hardware that it works
>> on supports that too.
> 
> It'd be nice to have some sort of feature parity, but it seems like a
> vanishingly small target of users having access to an ETM sink.
> 
>>
>> Apart from compilation and checking that the exclude guest settings
>> are correctly programmed on guest switch, this is untested by me.
> 
> I'll have a look at the series, but none of my HW fits in this
> description (my ARMv8.4+ boxes don't have any form of tracing).

While I have your attention, we have another series that manages the
trace filtering for Guests on VHE, completely within the Coresight etm4x
driver here [0]. I personally think, it is good to have the guest
filtering for both nVHE and VHE under the KVM control, like we do
in this series. I would like your opinion on this.

[0] https://lkml.kernel.org/r/20230804085219.260790-1-james.clark@arm.com


Suzuki


> 
> Thanks,
> 
> 	M.
>