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:   Tue, 5 May 2020 14:59:35 -0400
From:   Vivek Goyal <vgoyal@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>, x86@...nel.org,
        kvm@...r.kernel.org, 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>
Subject: Re: [PATCH RFC 6/6] KVM: x86: Switch KVM guest to using interrupts
 for page ready APF delivery

On Wed, Apr 29, 2020 at 12:53:33PM +0200, Paolo Bonzini wrote:
> 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)?

If we are planning to report errors using this interface, then retaining
KVM_PV_REASON_PAGE_READY makes sense because we can then introduce another
state say KVM_PV_REASON_PAGE_ERROR.

Thanks
Vivek