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From:   Jim Mattson <jmattson@google.com>
Date:   Thu, 6 Aug 2020 10:36:16 -0700
Message-ID: <CALMp9eRonv7Ds2fw_fByqpQcyQBuq3fEXNnzi6ZmEHd2nHYO+w@mail.gmail.com>
Subject: Re: [PATCH 2/3] KVM: x86: introduce KVM_MEM_PCI_HOLE memory
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        kvm list <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Peter Xu <peterx@redhat.com>,
        Julia Suvorova <jsuvorov@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Wed, Aug 5, 2020 at 5:18 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Wed, Aug 05, 2020 at 10:05:40AM -0700, Jim Mattson wrote:
> > On Tue, Jul 28, 2020 at 7:38 AM Vitaly Kuznetsov <vkuznets@redhat.com> 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 ...

Sometimes, maybe. That's not the way the emulation of most VMX
instructions works, should they access unbacked memory. Perhaps that's
just a whole slew of bugs to be fixed. :-)