linux-kernel - Re: [PATCH v14 00/11] KVM: x86/pmu: Guest Last Branch Recording Enabling

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Date:   Mon, 19 Sep 2022 11:08:28 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, ak@...ux.intel.com,
        wei.w.wang@...el.com, kan.liang@...el.com,
        alex.shi@...ux.alibaba.com, kvm@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v14 00/11] KVM: x86/pmu: Guest Last Branch Recording Enabling

On Mon, Sep 19, 2022 at 12:26 AM Like Xu <like.xu.linux@...il.com> wrote:
>
> On 14/9/2022 7:42 am, Jim Mattson wrote:
> > How does live migration work? I don't see any mechanism for recording
> > the current LBR MSRs on suspend or restoring them on resume.
>
> Considering that LBR is still a model specific feature, migration is less
> valuable unless
> both LBR_FMT values of the migration side are the same, the compatibility check
> (based on cpu models) is required (gathering dust in my to-do list);

This seems like a problem best solved in the control plane.

> and there is another dusty missing piece is how to ensure that vcpu can get LBR
> hardware in
> vmx transition when KVM lbr event fails in host lbr event competition, the
> complexity here is
> that the host and guest may have different LBR filtering options.

In case of a conflict, who currently wins? The host or the guest? I'd
like the guest to win, but others may feel differently. Maybe we need
a configuration knob?

> The good news is the Architecture LBR will add save/restore support since Paolo
> is not averse to
> putting more msr's into msrs_to_save_all[], perhaps a dynamic addition mechanism
> is a prerequisite.
>
> Please let me know what your priority preferences are for these tasks above.