linux-kernel - Re: [PATCH RFC] x86/kvm/lapic: always disable MMIO interface in x2APIC mode

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Message-ID: <87wot899qw.fsf@vitty.brq.redhat.com>
Date:   Thu, 02 Aug 2018 15:30:15 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Jim Mattson <jmattson@...gle.com>, kvm list <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC] x86/kvm/lapic: always disable MMIO interface in x2APIC mode

Paolo Bonzini <pbonzini@...hat.com> writes:

> On 30/07/2018 11:14, Vitaly Kuznetsov wrote:
>> Paolo Bonzini <pbonzini@...hat.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.
>> 
>
> FWIW it is possible to move the MSI memory region from system memory to
> the PCI address space in QEMU, however I'm worried about backwards
> compatibility.
>

You know better :-)

> Vitaly, perhaps you could resubmit this patch, and provide a
> KVM_CAP_DISABLE_QUIRKS switch that would make apic_mmio_{read,write}
> return -EOPNOTSUPP in this case?

Just to make sure I understand,

we introduce a KVM_QUIRK_LAPIC_DISABLED_MMIO bit and will be emulating
MMIO hole in KVM till Qemu is able to deal with reads/writes passed to
it correctly?

-- 
  Vitaly