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Date:   Wed, 5 Aug 2020 20:18:37 -0400
From:   "Michael S. Tsirkin" <>
To:     Jim Mattson <>
Cc:     Vitaly Kuznetsov <>,
        kvm list <>,
        Paolo Bonzini <>,
        Sean Christopherson <>,
        Wanpeng Li <>,
        Peter Xu <>,
        Julia Suvorova <>,
        Andy Lutomirski <>,
        LKML <>
Subject: Re: [PATCH 2/3] KVM: x86: introduce KVM_MEM_PCI_HOLE memory

On Wed, Aug 05, 2020 at 10:05:40AM -0700, Jim Mattson wrote:
> On Tue, Jul 28, 2020 at 7:38 AM Vitaly Kuznetsov <> wrote:
> >
> > PCIe config space can (depending on the configuration) be quite big but
> > usually is sparsely populated. Guest may scan it by accessing individual
> > device's page which, when device is missing, is supposed to have 'pci
> > hole' semantics: reads return '0xff' and writes get discarded. Compared
> > to the already existing KVM_MEM_READONLY, VMM doesn't need to allocate
> > real memory and stuff it with '0xff'.
> Note that the bus error semantics described should apply to *any*
> unbacked guest physical addresses, not just addresses in the PCI hole.
> (Typically, this also applies to the standard local APIC page
> (0xfee00xxx) when the local APIC is either disabled or in x2APIC mode,
> which is an area that kvm has had trouble with in the past.)

Well ATM from KVM's POV unbacked -> exit to userspace, right?
Not sure what you are suggesting here ...


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