On Wed, Aug 5, 2020 at 5:18 PM Michael S. Tsirkin <mst@xxxxxxxxxx> 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@xxxxxxxxxx> 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. :-)
