linux-kernel - Re: [PATCH RFC 6/6] KVM: x86: Switch KVM guest to using interrupts for page ready APF delivery

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Message-ID: <87blnah36e.fsf@vitty.brq.redhat.com>
Date:   Wed, 29 Apr 2020 14:44:25 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>, x86@...nel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH RFC 6/6] KVM: x86: Switch KVM guest to using interrupts for page ready APF delivery

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

> On 29/04/20 11:36, Vitaly Kuznetsov wrote:
>> +
>> +	if (__this_cpu_read(apf_reason.enabled)) {
>> +		reason = __this_cpu_read(apf_reason.reason);
>> +		if (reason == KVM_PV_REASON_PAGE_READY) {
>> +			token = __this_cpu_read(apf_reason.token);
>> +			/*
>> +			 * Make sure we read 'token' before we reset
>> +			 * 'reason' or it can get lost.
>> +			 */
>> +			mb();
>> +			__this_cpu_write(apf_reason.reason, 0);
>> +			kvm_async_pf_task_wake(token);
>> +		}
>
> If tokens cannot be zero, could we avoid using reason for the page ready
> interrupt (and ultimately retire "reason" completely)?

Yes, we can switch to using 'token' exclusively but personally I'm not
sure it is worth it. We'll still have to have a hole and reason + token
is only u64. Keeping 'reason' in place allows us to easily come up with
any other type of notification through this mecanism (if the reson is
... then 'token' means ...).

-- 
Vitaly