linux-kernel - Re: [PATCH v7 08/12] KVM/x86/vPMU: Add APIs to support host save/restore the guest lbr stack

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Date:   Mon, 8 Jul 2019 16:48:31 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        pbonzini@...hat.com, ak@...ux.intel.com, kan.liang@...el.com,
        mingo@...hat.com, rkrcmar@...hat.com, like.xu@...el.com,
        jannh@...gle.com, arei.gonglei@...wei.com, jmattson@...gle.com
Subject: Re: [PATCH v7 08/12] KVM/x86/vPMU: Add APIs to support host
 save/restore the guest lbr stack

On Mon, Jul 08, 2019 at 09:23:15AM +0800, Wei Wang wrote:
> From: Like Xu <like.xu@...el.com>
> 
> This patch adds support to enable/disable the host side save/restore

This patch should be disqualified on Changelog alone...

  Documentation/process/submitting-patches.rst:instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy

> for the guest lbr stack on vCPU switching. To enable that, the host
> creates a perf event for the vCPU, and the event attributes are set
> to the user callstack mode lbr so that all the conditions are meet in
> the host perf subsystem to save the lbr stack on task switching.
> 
> The host side lbr perf event are created only for the purpose of saving
> and restoring the lbr stack. There is no need to enable the lbr
> functionality for this perf event, because the feature is essentially
> used in the vCPU. So perf_event_create is invoked with need_counter=false
> to get no counter assigned for the perf event.
> 
> The vcpu_lbr field is added to cpuc, to indicate if the lbr perf event is
> used by the vCPU only for context switching. When the perf subsystem
> handles this event (e.g. lbr enable or read lbr stack on PMI) and finds
> it's non-zero, it simply returns.

*WHY* does the host need to save/restore? Why not make VMENTER/VMEXIT do
this?

Many of these patches don't explain why things are done; that's a
problem.