linux-kernel - Re: [PATCH 2/8] KVM: x86: extend struct kvm_vcpu_pv_apf

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Date:   Fri, 15 May 2020 17:59:43 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vivek Goyal <vgoyal@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        x86@...nel.org, 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>,
        Jim Mattson <jmattson@...gle.com>,
        Gavin Shan <gshan@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/8] KVM: x86: extend struct kvm_vcpu_pv_apf_data with
 token info

On 13/05/20 14:52, Vivek Goyal wrote:
>>> Also, type of event should not necessarily be tied to delivery method.
>>> For example if we end up introducing say, "KVM_PV_REASON_PAGE_ERROR", then
>>> I would think that event can be injected both using exception (#PF or #VE)
>>> as well as interrupt (depending on state of system).
>> Why bother preserving backwards compatibility?
> New machanism does not have to support old guests but old mechanism
> should probably continue to work and deprecated slowly, IMHO. Otherwise
> guests which were receiving async page faults will suddenly stop getting
> it over hypervisor upgrade and possibly see drop in performance.

Unfortunately, the old mechanism was broken enough, and in enough
different ways, that it's better to just drop it.

The new one using #VE is not coming very soon (we need to emulate it for
<Broadwell and AMD processors, so it's not entirely trivial) so we are
going to keep "page not ready" delivery using #PF for some time or even
forever.  However, page ready notification as #PF is going away for good.

That said, type1/type2 is quite bad. :)  Let's change that to page not
present / page ready.

Thanks,

Paolo