linux-kernel - Re: [PATCH 2/6] kvm: x86: drop read_tsc

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Message-ID: <6d755392-d0f7-078f-8db2-b6539846d709@redhat.com>
Date:   Wed, 21 Sep 2016 17:22:14 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Luiz Capitulino <lcapitulino@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        rostedt@...dmis.org, mhiramat@...nel.org, mtosatti@...hat.com
Subject: Re: [PATCH 2/6] kvm: x86: drop read_tsc_offset()



On 21/09/2016 17:19, Jim Mattson wrote:
> Doesn't that assume you can run the merge program in L1?

You only need the TSC offset, but we should make sure that L0
tracepoints contain enough information to figure out the L0->L2 TSC
offsets (they are the values in VMCS02).

That said, how would you get the trace from L1 if you don't have access
to it?

Paolo

> On Mon, Sep 19, 2016 at 10:37 PM, Paolo Bonzini <pbonzini@...hat.com> wrote:
>>
>>
>> On 20/09/2016 00:18, Jim Mattson wrote:
>>> Hmmm. Yes, I think it does. With this patch series,
>>> vcpu->arch.tsc_offset appears to contain L1's TSC offset (perhaps
>>> making vmx->nested.vmcs01_tsc_offset redundant).
>>>
>>> However, this unfortunately limits the newly added functionality to
>>> merging host and *L1* guest traces. It doesn't work with L2 (or
>>> deeper) guests. Or perhaps I'm missing something?
>>
>> You can merge L1/L2 first and then host/L1.
>>
>> Paolo