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

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Date:   Wed, 29 Apr 2020 15:19:15 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...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

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.

Thanks,

Paolo