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

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Date:   Mon, 19 Sep 2022 15:26:01 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Jim Mattson <jmattson@...gle.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 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);

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.

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.