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

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Date:   Thu, 3 May 2018 14:31:01 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:     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 03/05/2018 14:09, Alexander Shishkin wrote:
>>> 1. Do we have an event in pt_ctx?
>>>  * No -> Set up the context for VMX.
>>>  * Yes -> 2. Is attr.exclude_guest set?
>>>              * No -> Guest trace goes to the host's buffer, do nothing.
>>> 	     * Yes -> Set up/switch the context for VMX.
>> Can you explain this more clearly?
> Let's see; in the intel_pt driver we have a per-cpu PT "context", from which
> we can tell if there is a host event that wants to trace the guest. This
> should provide enough information to make a decision whether we want to
> context switch PT MSRs or not on the spot, instead of having a module
> parameter.

I still don't understand how the host event is useful...

Paolo