linux-kernel - Re: [PATCH 2/3] KVM: x86: introduce KVM_MEM_PCI

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Date:   Thu, 06 Aug 2020 11:14:46 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Peter Xu <peterx@...hat.com>, Michael Tsirkin <mst@...hat.com>,
        Julia Suvorova <jsuvorov@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] KVM: x86: introduce KVM_MEM_PCI_HOLE memory

Jim Mattson <jmattson@...gle.com> writes:

> On Tue, Jul 28, 2020 at 7:38 AM Vitaly Kuznetsov <vkuznets@...hat.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