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: <871ro6gy30.fsf@vitty.brq.redhat.com>
Date:   Wed, 29 Apr 2020 16:34:27 +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 14:44, Vitaly Kuznetsov wrote:
>>>> +			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 ...).
>
> If we need a "reason" field I'd rather make it separate from the page
> not ready reason, because as we differentiate the delivery mechanism it
> is cleaner to keep them separate.
>
> For example, if the reason is present but separate, the memory barrier
> is not necessary anymore, because apf_reason.token cannot be written
> before the ack MSR is written.  And with #VE there will be already a
> hardware-provided mechanism to avoid reentrancy.

Ok, makes sense. I'll probably use your original idea and use 'token'
for 'page ready' notification exclusively for now. In case of need we
can always extend 'struct kvm_vcpu_pv_apf_data' with the information we
need so we can avoid adding 'reason2' for now.

-- 
Vitaly