linux-kernel - Re: [PATCH v4 2/7] KVM: X86: Enable fastpath when APICv is enabled

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Date:   Mon, 4 May 2020 19:06:53 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Haiwei Li <lihaiwei@...cent.com>
Subject: Re: [PATCH v4 2/7] KVM: X86: Enable fastpath when APICv is enabled

On 30/04/20 15:34, Vitaly Kuznetsov wrote:
>>  static enum exit_fastpath_completion vmx_exit_handlers_fastpath(struct kvm_vcpu *vcpu)
>>  {
>> -	if (!is_guest_mode(vcpu)) {
>> +	if (!is_guest_mode(vcpu) && vcpu->arch.apicv_active) {
>>  		switch (to_vmx(vcpu)->exit_reason) {
>>  		case EXIT_REASON_MSR_WRITE:
>>  			return handle_fastpath_set_msr_irqoff(vcpu);
> I think that apicv_active checks are specific to APIC MSRs but
> handle_fastpath_set_msr_irqoff() can handle any other MSR as well. I'd
> suggest to move the check inside handle_fastpath_set_msr_irqoff().
> 
> Also, enabling Hyper-V SynIC leads to disabling apicv. It it still
> pointless to keep fastpath enabled?

Indeed, only fast paths that only apply to apicv should be disabled (and
ideally there should be a WARN_ON in the code that doesn't support !apicv).

Paolo