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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Jim Mattson <jmattson@google.com>, kvm list <kvm@vger.kernel.org>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH RFC] x86/kvm/lapic: always disable MMIO interface in x2APIC mode
References: <20180727144448.9606-1-vkuznets@redhat.com>
        <CALMp9eSaiw7wMMhzKZQpM141h6zEz=shu5P++BerbQBTVyFdYA@mail.gmail.com>
        <e3b7080f-b788-048b-6f3a-e1a90e4206b7@redhat.com>
Date:   Mon, 30 Jul 2018 11:14:54 +0200
In-Reply-To: <e3b7080f-b788-048b-6f3a-e1a90e4206b7@redhat.com> (Paolo
        Bonzini's message of "Fri, 27 Jul 2018 19:00:38 +0200")
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Paolo Bonzini <pbonzini@redhat.com> writes:

> On 27/07/2018 18:48, Jim Mattson wrote:
>> On a physical machine, I would expect the default local APIC page to
>> fall in the PCI hole, so it would be correct to sink writes and to
>> return all ones for reads. Does qemu implement a PCI hole, and does
>> this address fall into it?
>
> It does implement a PCI hole, but when using the kernel LAPIC it expects
> that only devices write to that range; therefore that address doesn't
> fall into the PCI hole, and instead it generates an MSIs.

Yes, and that's why I believe it's correct to never forward lapic
reads/writes from KVM to userspace when lapic is in kernel.

"RFC" was mostly about the inconsistency with the case when APIC access
page is in use. To be 100% correct I would suggest to somehow make it
behave like MMIO hole in case we're in x2APIC/disabled mode too.

-- 
  Vitaly