linux-kernel - Re: [PATCH v2 06/10] KVM: x86: acknowledgment mechanism for async pf page ready notifications

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Date:   Thu, 28 May 2020 13:39:21 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        x86@...nel.org
Cc:     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>,
        Vivek Goyal <vgoyal@...hat.com>, Gavin Shan <gshan@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 06/10] KVM: x86: acknowledgment mechanism for async pf page ready notifications

Paolo Bonzini <pbonzini@...hat.com> writes:

> On 25/05/20 16:41, Vitaly Kuznetsov wrote:
>> +	case MSR_KVM_ASYNC_PF_ACK:
>> +		if (data & 0x1) {
>> +			vcpu->arch.apf.pageready_pending = false;
>> +			kvm_check_async_pf_completion(vcpu);
>> +		}
>> +		break;
>>  	case MSR_KVM_STEAL_TIME:
>>  
>>  		if (unlikely(!sched_info_on()))
>> @@ -3183,6 +3189,9 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>  	case MSR_KVM_ASYNC_PF_INT:
>>  		msr_info->data = vcpu->arch.apf.msr_int_val;
>>  		break;
>> +	case MSR_KVM_ASYNC_PF_ACK:
>> +		msr_info->data = 0;
>> +		break;
>
> How is the pageready_pending flag migrated?  Should we revert the
> direction of the MSR (i.e. read the flag, and write 0 to clear it)?

The flag is not migrated so it will be 'false'. This can just cause an
extra kick in kvm_arch_async_page_present_queued() but this shouldn't be
a big deal. Also, after migration we will just send 'wakeup all' event,
async pf queue will be empty. MSR_KVM_ASYNC_PF_ACK by itself is not
migrated, we don't even store it, not sure how invering it would change
things.

-- 
Vitaly