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

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Date:   Mon, 19 Sep 2016 15:18:02 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.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()

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?

On Mon, Sep 19, 2016 at 8:34 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
>
>
> On 19/09/2016 17:30, Jim Mattson wrote:
>> vmx_read_tsc_offset has a bug when running nested VMs.  It should really be:
>>
>>        if (is_guest_mode(vcpu))
>>                return to_vmx(vcpu)->nested.vmcs01_tsc_offset;
>>        else
>>                return vmcs_read64(TSC_OFFSET);
>>
>> Perhaps a better name woulf be "vmx_get_l1_tsc_offset."
>
> I agree, but doesn't this patch fix the bug too?
>
> Paolo
>
>> In any case, this does not seem consistent with vcpu->arch.tsc_offset.
>