linux-kernel - Re: [PATCH v7 06/13] KVM: x86: Add Intel Processor Trace virtualization mode

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Date:   Thu, 3 May 2018 13:50:39 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Luwei Kang <luwei.kang@...el.com>
Cc:     kvm@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        hpa@...or.com, x86@...nel.org, rkrcmar@...hat.com,
        linux-kernel@...r.kernel.org, joro@...tes.org,
        peterz@...radead.org, chao.p.peng@...ux.intel.com
Subject: Re: [PATCH v7 06/13] KVM: x86: Add Intel Processor Trace
 virtualization mode

On 03/05/2018 13:32, Alexander Shishkin wrote:
>>  
>> +/* Default is SYSTEM mode. */
>> +static int __read_mostly pt_mode = PT_MODE_SYSTEM;
>> +module_param(pt_mode, int, S_IRUGO);
> So, it's an explicit module parameter? One apparent problem with this
> is that one would need to reload kvm module(s) to be able to use PT,
> which is not ideal.

If you want to do tracing system-wide, that by definition must disable
guest tracing, so I think the module parameter is appropriate.  The
question is more, what is the best default.

Paolo