linux-kernel - Re: [PATCH v4 13/17] kvm: i8254: Deactivate APICv when using in-kernel PIT re-injection mode.

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Date:   Mon, 4 Nov 2019 18:54:57 +0000
From:   "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC:     "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "rkagan@...tuozzo.com" <rkagan@...tuozzo.com>,
        "graf@...zon.com" <graf@...zon.com>,
        "jschoenh@...zon.de" <jschoenh@...zon.de>,
        "karahmed@...zon.de" <karahmed@...zon.de>,
        "rimasluk@...zon.com" <rimasluk@...zon.com>,
        "Grimm, Jon" <Jon.Grimm@....com>
Subject: Re: [PATCH v4 13/17] kvm: i8254: Deactivate APICv when using
 in-kernel PIT re-injection mode.

Paolo,

Thanks for quick response.

On 11/2/19 4:57 AM, Paolo Bonzini wrote:
> On 01/11/19 23:41, Suthikulpanit, Suravee wrote:
>> +	/*
>> +	 * AMD SVM AVIC accelerates EOI write and does not trap.
>> +	 * This cause in-kernel PIT re-inject mode to fail
>> +	 * since it checks ps->irq_ack before kvm_set_irq()
>> +	 * and relies on the ack notifier to timely queue
>> +	 * the pt->worker work iterm and reinject the missed tick.
>> +	 * So, deactivate APICv when PIT is in reinject mode.
>> +	 */
>>   	if (reinject) {
>> +		kvm_request_apicv_update(kvm, false, APICV_DEACT_BIT_PIT_REINJ);
>>   		/* The initial state is preserved while ps->reinject == 0. */
>>   		kvm_pit_reset_reinject(pit);
>>   		kvm_register_irq_ack_notifier(kvm, &ps->irq_ack_notifier);
>>   		kvm_register_irq_mask_notifier(kvm, 0, &pit->mask_notifier);
>>   	} else {
>> +		kvm_request_apicv_update(kvm, true, APICV_DEACT_BIT_PIT_REINJ);
>>   		kvm_unregister_irq_ack_notifier(kvm, &ps->irq_ack_notifier);
>>   		kvm_unregister_irq_mask_notifier(kvm, 0, &pit->mask_notifier);
> 
> This is not too nice for Intel which does support (through the EOI exit
> mask) APICv even if PIT reinjection active.

I see you point.

> We can work around it by adding a global mask of inhibit reasons that
> apply to the vendor, and initializing it as soon as possible in vmx.c/svm.c.
> 
> Then kvm_request_apicv_update can ignore reasons that the vendor doesn't
> care about.
> 
> Paolo
> 

What about we enhance the pre_update_apivc_exec_ctrl() to also return 
success/fail. In here, the vendor specific code can decide to update 
APICv state or not.

Thanks,
Suravee