linux-kernel - Re: [RFC PATCH 23/41] KVM: x86/pmu: Implement the save/restore of PMU state for Intel CPU

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Date: Wed, 24 Apr 2024 08:00:23 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Dapeng Mi <dapeng1.mi@...ux.intel.com>
Cc: Mingwei Zhang <mizhang@...gle.com>, maobibo <maobibo@...ngson.cn>, 
	Xiong Zhang <xiong.y.zhang@...ux.intel.com>, pbonzini@...hat.com, peterz@...radead.org, 
	kan.liang@...el.com, zhenyuw@...ux.intel.com, jmattson@...gle.com, 
	kvm@...r.kernel.org, linux-perf-users@...r.kernel.org, 
	linux-kernel@...r.kernel.org, zhiyuan.lv@...el.com, eranian@...gle.com, 
	irogers@...gle.com, samantha.alt@...el.com, like.xu.linux@...il.com, 
	chao.gao@...el.com
Subject: Re: [RFC PATCH 23/41] KVM: x86/pmu: Implement the save/restore of PMU
 state for Intel CPU

On Wed, Apr 24, 2024, Dapeng Mi wrote:
> 
> On 4/24/2024 1:02 AM, Mingwei Zhang wrote:
> > > > Maybe, (just maybe), it is possible to do PMU context switch at vcpu
> > > > boundary normally, but doing it at VM Enter/Exit boundary when host is
> > > > profiling KVM kernel module. So, dynamically adjusting PMU context
> > > > switch location could be an option.
> > > If there are two VMs with pmu enabled both, however host PMU is not
> > > enabled. PMU context switch should be done in vcpu thread sched-out path.
> > > 
> > > If host pmu is used also, we can choose whether PMU switch should be
> > > done in vm exit path or vcpu thread sched-out path.
> > > 
> > host PMU is always enabled, ie., Linux currently does not support KVM
> > PMU running standalone. I guess what you mean is there are no active
> > perf_events on the host side. Allowing a PMU context switch drifting
> > from vm-enter/exit boundary to vcpu loop boundary by checking host
> > side events might be a good option. We can keep the discussion, but I
> > won't propose that in v2.
> 
> I suspect if it's really doable to do this deferring. This still makes host
> lose the most of capability to profile KVM. Per my understanding, most of
> KVM overhead happens in the vcpu loop, exactly speaking in VM-exit handling.
> We have no idea when host want to create perf event to profile KVM, it could
> be at any time.

No, the idea is that KVM will load host PMU state asap, but only when host PMU
state actually needs to be loaded, i.e. only when there are relevant host events.

If there are no host perf events, KVM keeps guest PMU state loaded for the entire
KVM_RUN loop, i.e. provides optimal behavior for the guest.  But if a host perf
events exists (or comes along), the KVM context switches PMU at VM-Enter/VM-Exit,
i.e. lets the host profile almost all of KVM, at the cost of a degraded experience
for the guest while host perf events are active.

My original sketch: https://lore.kernel.org/all/ZR3eNtP5IVAHeFNC@google.com