linux-kernel - Re: [PATCH v4 08/17] kvm: x86: Introduce APICv pre-update hook

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Message-ID: <20191112111215.GB2397@rkaganb.sw.ru>
Date:   Tue, 12 Nov 2019 11:12:19 +0000
From:   Roman Kagan <rkagan@...tuozzo.com>
To:     Suravee Suthikulpanit <suravee.suthikulpanit@....com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "graf@...zon.com" <graf@...zon.com>,
        "jschoenh@...zon.de" <jschoenh@...zon.de>,
        "karahmed@...zon.de" <karahmed@...zon.de>,
        "rimasluk@...zon.com" <rimasluk@...zon.com>,
        "Grimm, Jon" <Jon.Grimm@....com>
Subject: Re: [PATCH v4 08/17] kvm: x86: Introduce APICv pre-update hook

On Mon, Nov 11, 2019 at 06:08:05PM -0600, Suravee Suthikulpanit wrote:
> On 11/4/19 4:05 PM, Roman Kagan wrote:
> > On Fri, Nov 01, 2019 at 10:41:31PM +0000, Suthikulpanit, Suravee wrote:
> > > AMD SVM AVIC needs to update APIC backing page mapping before changing
> > > APICv mode. Introduce struct kvm_x86_ops.pre_update_apicv_exec_ctrl
> > > function hook to be called prior KVM APICv update request to each vcpu.
> > This again seems to mix up APIC backing page and APIC access page.
> > 
> > And I must be missing something obvious, but why is it necessary to
> > unmap the APIC access page while AVIC is disabled?  Does keeping it
> > around stand in the way when working with AVIC disabled?
> 
> I have replied to patch 07/17 with explanation.
> 
> Yes, keeping the APIC access page while disabling AVIC would cause
> the SVM to not function properly.

I wonder why?  Once AVIC is disabled guest access to this page would
trigger a regular NPT fault vmexit, just as it would with the NPT entry
for this page destroyed, wouldn't it?  So there would be no difference
from the host's POV.  Am I missing something?

Thanks,
Roman.