linux-kernel - Re: [PATCH v5 8/8] KVM: VMX: Resize PID-ponter table on demand for IPI virtualization

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Message-ID: <Yei0d0KVnNphPrP3@google.com>
Date:   Thu, 20 Jan 2022 01:01:43 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Zeng Guang <guang.zeng@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Kim Phillips <kim.phillips@....com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Jethro Beekman <jethro@...tanix.com>,
        "Huang, Kai" <kai.huang@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Hu, Robert" <robert.hu@...el.com>,
        "Gao, Chao" <chao.gao@...el.com>
Subject: Re: [PATCH v5 8/8] KVM: VMX: Resize PID-ponter table on demand for
 IPI virtualization

On Wed, Jan 19, 2022, Zeng Guang wrote:
> It's self-adaptive , standalone function module in kvm, no any extra
> limitation introduced

I disagree.  Its failure mode on OOM is to degrade guest performance, _that_ is
a limitation.  OOM is absolutely something that should be immediately communicated
to userspace in a way that userspace can take action.

> and scalable even future extension on KVM_MAX_VCPU_IDS or new apic id
> implementation released.
>
> How do you think ? :)

Heh, I think I've made it quite clear that I think it's unnecesary complexity in
KVM.  It's not a hill I'll die on, e.g. if Paolo and others feel it's the right
approach then so be it, but I really, really dislike the idea of dynamically
changing the table, KVM has a long and sordid history of botching those types
of flows/features.