linux-kernel - Re: [PATCH v3 06/12] KVM: x86: don't disable APICv memslot when inhibited

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Date:   Tue, 3 Aug 2021 10:44:09 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...en8.de>,
        Sean Christopherson <seanjc@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v3 06/12] KVM: x86: don't disable APICv memslot when
 inhibited

Reviewing this patch and the next one together.

On 02/08/21 20:33, Maxim Levitsky wrote:
> +static int avic_alloc_access_page(struct kvm *kvm)
>  {
>  	void __user *ret;
>  	int r = 0;
>  
>  	mutex_lock(&kvm->slots_lock);
> +
> +	if (kvm->arch.apic_access_memslot_enabled)
>  		goto out;

This variable is overloaded between "is access enabled" and "is the 
memslot allocated".  I think you should check 
kvm->arch.apicv_inhibit_reasons instead in kvm_faultin_pfn.


> +	if (!activate)
> +		kvm_zap_gfn_range(kvm, gpa_to_gfn(APIC_DEFAULT_PHYS_BASE),
> +				  gpa_to_gfn(APIC_DEFAULT_PHYS_BASE + PAGE_SIZE));
> +

Off by one, the last argument of kvm_zap_gfn_range is inclusive:
Also, checking "activate" is a bit ugly when we have "new" available as 
well.  Yes, they are the same if !!old != !!new, but we care about the 
global state, not the single bit.

Putting everything together, this could become something like

         trace_kvm_apicv_update_request(activate, bit);
         if (!!old != !!new) {
		/*
		 * Kick all CPUs out of guest mode.  When
		 * kvm_vcpu_update_apicv succeeds in taking
		 * apicv_update_lock, it will see the
		 * new apicv_inhibit_reasons that we set below.
		 */
	        kvm_make_all_cpus_request(kvm, KVM_REQ_APICV_UPDATE);

	        if (new) {
	                unsigned long gfn = gpa_to_gfn(APIC_DEFAULT_PHYS_BASE);
	                kvm_zap_gfn_range(kvm, gfn, gfn);
	        }
	}
         kvm->arch.apicv_inhibit_reasons = new;
         mutex_unlock(&kvm->arch.apicv_update_lock);

Paolo