linux-kernel - Re: [PATCH] x86/kvm: Introduce boot parameter no-kvm-pvipi

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Date:   Wed, 20 Oct 2021 20:12:04 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     zhenwei pi <pizhenwei@...edance.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Wanpeng Li <wanpengli@...cent.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org
Subject: Re: [PATCH] x86/kvm: Introduce boot parameter no-kvm-pvipi

On Wed, Oct 20, 2021, Wanpeng Li wrote:
> On Wed, 20 Oct 2021 at 20:08, zhenwei pi <pizhenwei@...edance.com> wrote:
> >
> > Although host side exposes KVM PV SEND IPI feature to guest side,
> > guest should still have a chance to disable it.
> >
> > A typicall case of this parameter:
> > If the host AMD server enables AVIC feature, the flat mode of APIC
> > get better performance in the guest.
> 
> Hmm, I didn't find enough valuable information in your posting. We
> observe AMD a lot before.
> https://lore.kernel.org/all/CANRm+Cx597FNRUCyVz1D=B6Vs2GX3Sw57X7Muk+yMpi_hb+v1w@mail.gmail.com/T/#u

I too would like to see numbers.  I suspect the answer is going to be that
AVIC performs poorly in CPU overcommit scenarios because of the cost of managing
the tables and handling "failed delivery" exits, but that AVIC does quite well
when vCPUs are pinned 1:1 and IPIs rarely require an exit to the host.