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Subject: Re: [PATCH 03/19] Revert "KVM: SVM: Introduce hybrid-AVIC mode"
From:   Maxim Levitsky <mlevitsk@redhat.com>
To:     Sean Christopherson <seanjc@google.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>,
        Li RongQing <lirongqing@baidu.com>
Date:   Wed, 31 Aug 2022 20:47:53 +0300
In-Reply-To: <Yw+KDoSSKN0xgJW0@google.com>
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         <20220831003506.4117148-4-seanjc@google.com>
         <17e776dccf01e03bce1356beb8db0741e2a13d9a.camel@redhat.com>
         <Yw+KDoSSKN0xgJW0@google.com>
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On Wed, 2022-08-31 at 16:19 +0000, Sean Christopherson wrote:
> On Wed, Aug 31, 2022, Maxim Levitsky wrote:
> > On Wed, 2022-08-31 at 00:34 +0000, Sean Christopherson wrote:
> > > Remove SVM's so called "hybrid-AVIC mode" and reinstate the restriction
> > > where AVIC is disabled if x2APIC is enabled.  The argument that the
> > > "guest is not supposed to access xAPIC mmio when uses x2APIC" is flat out
> > > wrong.  Activating x2APIC completely disables the xAPIC MMIO region,
> > > there is nothing that says the guest must not access that address.
> > > 
> > > Concretely, KVM-Unit-Test's existing "apic" test fails the subtests that
> > > expect accesses to the APIC base region to not be emulated when x2APIC is
> > > enabled.
> > > 
> > > Furthermore, allowing the guest to trigger MMIO emulation in a mode where
> > > KVM doesn't expect such emulation to occur is all kinds of dangerous.
> > > 
> > > Tweak the restriction so that it only inhibits AVIC if x2APIC is actually
> > > enabled instead of inhibiting AVIC is x2APIC is exposed to the guest.
> > > 
> > > This reverts commit 0e311d33bfbef86da130674e8528cc23e6acfe16.
> > 
> > I don't agree with this patch.
> > 
> > When reviewing this code I did note that MMIO is left enabled which is kind
> > of errata on KVM side, and nobody objected to this.
> 
> I didn't object because I didn't read the patch.  I'm very much objecting now.
> 

And I am *very* much objecting to reverting this patch.

Best regards,
	Maxim Levitsky