linux-kernel - Re: [PATCH] KVM: x86: use TPAUSE to replace PAUSE in halt polling

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Message-ID: <f1598980-92a8-267c-cade-8f62d7653017@redhat.com>
Date:   Wed, 24 Aug 2022 19:19:08 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Dapeng Mi <dapeng1.mi@...el.com>
Cc:     rafael@...nel.org, daniel.lezcano@...aro.org,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, zhenyuw@...ux.intel.com
Subject: Re: [PATCH] KVM: x86: use TPAUSE to replace PAUSE in halt polling

On 8/24/22 17:26, Sean Christopherson wrote:
> I say "if", because I think this needs to come with performance numbers to show
> the impact on guest latency so that KVM and its users can make an informed decision.
> And if it's unlikely that anyone will ever want to enable TPAUSE for halt polling,
> then it's not worth the extra complexity in KVM.

Yeah, halt polling works around perhaps the biggest performance issue 
with VMs compared to bare metal (so much that it's even possible to move 
halt polling _inside_ the guest for extra performance).

I am ready to be proven wrong but I doubt TPAUSE will have a small 
effect, and if one wants the most power saving they should disable halt 
polling.  Perhaps KVM could do it automatically if the powersaving 
governor is in effect?

Paolo