linux-kernel - Re: [PATCH v2 05/10] KVM: x86: interrupt based APF 'page ready' event delivery

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Date:   Tue, 9 Jun 2020 15:10:35 -0400
From:   Vivek Goyal <vgoyal@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Jim Mattson <jmattson@...gle.com>,
        Gavin Shan <gshan@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 05/10] KVM: x86: interrupt based APF 'page ready'
 event delivery

On Mon, May 25, 2020 at 04:41:20PM +0200, Vitaly Kuznetsov wrote:
[..]
>  void kvm_arch_async_page_present(struct kvm_vcpu *vcpu,
>  				 struct kvm_async_pf *work)
>  {
> -	struct x86_exception fault;
> +	struct kvm_lapic_irq irq = {
> +		.delivery_mode = APIC_DM_FIXED,
> +		.vector = vcpu->arch.apf.vec
> +	};
>  
>  	if (work->wakeup_all)
>  		work->arch.token = ~0; /* broadcast wakeup */
> @@ -10444,26 +10491,20 @@ void kvm_arch_async_page_present(struct kvm_vcpu *vcpu,
>  		kvm_del_async_pf_gfn(vcpu, work->arch.gfn);
>  	trace_kvm_async_pf_ready(work->arch.token, work->cr2_or_gpa);
>  
> -	if (vcpu->arch.apf.msr_val & KVM_ASYNC_PF_ENABLED &&
> -	    !apf_put_user_ready(vcpu, work->arch.token)) {
> -			fault.vector = PF_VECTOR;
> -			fault.error_code_valid = true;
> -			fault.error_code = 0;
> -			fault.nested_page_fault = false;
> -			fault.address = work->arch.token;
> -			fault.async_page_fault = true;
> -			kvm_inject_page_fault(vcpu, &fault);
> -	}
> +	if (kvm_pv_async_pf_enabled(vcpu) &&
> +	    !apf_put_user_ready(vcpu, work->arch.token))
> +		kvm_apic_set_irq(vcpu, &irq, NULL);
> +

Hi Vitaly,

Have a question about page ready events. 

Now we deliver PAGE_NOT_PRESENT page faults only if guest is not in
kernel mode. So say kernel tried to access a page and we halted cpu.
When page is available, we will inject page_ready interrupt. At
that time we don't seem to check whether page_not_present was injected
or not. 

IOW, we seem to deliver page_ready irrespective of the fact whether
PAGE_NOT_PRESENT was delivered or not. And that means we will be
sending page present tokens to guest. Guest will not have a state
associated with that token and think that page_not_present has
not been delivered yet and allocate an element in hash table for
future page_not_present event. And that will lead to memory leak
and token conflict etc.

While setting up async pf, should we keep track whether associated
page_not_present was delivered to guest or not and deliver page_ready
accordingly.

Thanks
Vivek