linux-kernel - Re: [PATCH 6/6] KVM: VMX: enable IPI virtualization

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Date:   Mon, 19 Jul 2021 21:16:59 +0800
From:   Zeng Guang <guang.zeng@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.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>
Cc:     "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>,
        Emanuele Giuseppe Esposito <eesposit@...hat.com>
Subject: Re: [PATCH 6/6] KVM: VMX: enable IPI virtualization

On 7/19/2021 4:32 AM, Paolo Bonzini wrote:
> On 17/07/21 05:55, Zeng Guang wrote:
>>>> +    if (vmx->ipiv_active)
>>>> +        install_pid(vmx);
>>> This should be if (enable_ipiv) instead, I think.
>>>
>>> In fact, in all other places that are using vmx->ipiv_active, you can
>>> actually replace it with enable_ipiv; they are all reached only with
>>> kvm_vcpu_apicv_active(vcpu) == true.
>>>
>> enable_ipiv as a global variable indicates the hardware capability to
>> enable IPIv. Each VM may have different IPIv configuration according to
>> kvm_vcpu_apicv_active status. So we use ipiv_active per VM to enclose
>> IPIv related operations.
> Understood, but in practice all uses of vmx->ipiv_active are guarded by
> kvm_vcpu_apicv_active so they are always reached with vmx->ipiv_active
> == enable_ipiv.
>
> The one above instead seems wrong and should just use enable_ipiv.
enable_ipiv associate with "IPI virtualization" setting in tertiary exec 
controls and
enable_apicv which depends on cpu_has_vmx_apicv(). kvm_vcpu_apicv_active 
still
can be false even if enable_ipiv is true, e.g. in case irqchip not 
emulated in kernel.

Though it's ok to use enable_ipiv here, vmcs setup succeed for IPIv but 
it won't take into effect as
false kvm_vcpu_apicv_active runtime disable “IPI virtualization" for VM 
in this case and
leads vmx->ipiv_active become false as well. So vmx->ipiv_active is more 
accurate to reflect
runtime IPIv status.
> Paolo
>