Re: [PATCH 2/3] KVM: x86: introduce KVM_MEM_PCI_HOLE memory

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Jim Mattson <jmattson@google.com> writes:

> 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.)

Yes, we can make KVM return 0xff on all read access to unbacked memory,
not only KVM_MEM_PCI_HOLE slots (and drop them completely). This,
however, takes the control from userspace: with KVM_MEM_PCI_HOLE
memslots we're saying 'accessing this unbacked memory is fine' and we
can still trap accesses to all other places. This should help in
detecting misbehaving guests.

-- 
Vitaly