linux-kernel - Re: [PATCH 7/7] cpuidle/poll_state: replace cpu_relax with smp_cond_load

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Date:   Wed, 22 Nov 2023 12:51:19 -0800 (PST)
From:   Christoph Lameter <cl@...ux.com>
To:     Mihai Carabas <mihai.carabas@...cle.com>
cc:     linux-arm-kernel@...ts.infradead.org, kvm@...r.kernel.org,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        catalin.marinas@....com, will@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, x86@...nel.org, hpa@...or.com,
        pbonzini@...hat.com, wanpengli@...cent.com, vkuznets@...hat.com,
        rafael@...nel.org, daniel.lezcano@...aro.org,
        akpm@...ux-foundation.org, pmladek@...e.com, peterz@...radead.org,
        dianders@...omium.org, npiggin@...il.com,
        rick.p.edgecombe@...el.com, joao.m.martins@...cle.com,
        juerg.haefliger@...onical.com, mic@...ikod.net, arnd@...db.de,
        ankur.a.arora@...cle.com
Subject: Re: [PATCH 7/7] cpuidle/poll_state: replace cpu_relax with
 smp_cond_load_relaxed


On Mon, 20 Nov 2023, Mihai Carabas wrote:

> cpu_relax on ARM64 does a simple "yield". Thus we replace it with
> smp_cond_load_relaxed which basically does a "wfe".

Well it clears events first (which requires the first WFE) and then does a 
WFE waiting for any events if no events were pending.

WFE does not cause a VMEXIT? Or does the inner loop of 
smp_cond_load_relaxed now do 2x VMEXITS?

KVM ARM64 code seems to indicate that WFE causes a VMEXIT. See 
kvm_handle_wfx().