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

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Message-ID: <221c297b-1270-371b-70a3-4cff4bed7a7e@redhat.com>
Date:   Thu, 3 May 2018 14:30:25 +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 14:02, Alexander Shishkin wrote:
> 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.

Guest tracing can only be enabled at boot time, because the guest's
CPUID changes depending on whether it's enabled.  And likewise if perf
record can do system-wide tracing at any time during the guest's
execution, we need to know it at boot time in order to set the guest CPUID.

Paolo