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

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Message-ID: <20180503120215.dev7ti43zk7yoovn@um.fi.intel.com>
Date:   Thu, 3 May 2018 15:02:15 +0300
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Luwei Kang <luwei.kang@...el.com>, 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 Thu, May 03, 2018 at 01:50:39PM +0200, Paolo Bonzini wrote:
> 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,

Sure.

> so I think the module parameter is appropriate.

I don't see why. System-wide tracing takes place while perf record is
running. When it's done, it's done and we can un-disable the guest
tracing, without requiring the user to kill all their VMs and reload
modules.

Regards,
--
Alex