This is a continuation of "[PATCH RFC 0/5] KVM: x86: KVM_MEM_ALLONES memory" work: https://lore.kernel.org/kvm/20200514180540.52407-1-vkuznets@xxxxxxxxxx/ and pairs with Julia's "x86/PCI: Use MMCONFIG by default for KVM guests": https://lore.kernel.org/linux-pci/20200722001513.298315-1-jusual@xxxxxxxxxx/ 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. When testing Linux kernel boot with QEMU q35 VM and direct kernel boot I observed 8193 accesses to PCI hole memory. When such exit is handled in KVM without exiting to userspace, it takes roughly 0.000001 sec. Handling the same exit in userspace is six times slower (0.000006 sec) so the overal; difference is 0.04 sec. This may be significant for 'microvm' ideas. Note, the same speed can already be achieved by using KVM_MEM_READONLY but doing this would require allocating real memory for all missing devices and e.g. 8192 pages gives us 32mb. This will have to be allocated for each guest separately and for 'microvm' use-cases this is likely a no-go. Introduce special KVM_MEM_PCI_HOLE memory: userspace doesn't need to back it with real memory, all reads from it are handled inside KVM and return '0xff'. Writes still go to userspace but these should be extremely rare. The original 'KVM_MEM_ALLONES' idea had additional optimizations: KVM was mapping all 'PCI hole' pages to a single read-only page stuffed with 0xff. This is omitted in this submission as the benefits are unclear: KVM will have to allocate SPTEs (either on demand or aggressively) and this also consumes time/memory. We can always take a look at possible optimizations later. Vitaly Kuznetsov (3): KVM: x86: move kvm_vcpu_gfn_to_memslot() out of try_async_pf() KVM: x86: introduce KVM_MEM_PCI_HOLE memory KVM: selftests: add KVM_MEM_PCI_HOLE test Documentation/virt/kvm/api.rst | 19 ++- arch/x86/include/uapi/asm/kvm.h | 1 + arch/x86/kvm/mmu/mmu.c | 19 +-- arch/x86/kvm/mmu/paging_tmpl.h | 10 +- arch/x86/kvm/x86.c | 10 +- include/linux/kvm_host.h | 7 +- include/uapi/linux/kvm.h | 3 +- tools/testing/selftests/kvm/Makefile | 1 + .../testing/selftests/kvm/include/kvm_util.h | 1 + tools/testing/selftests/kvm/lib/kvm_util.c | 81 +++++++------ .../kvm/x86_64/memory_slot_pci_hole.c | 112 ++++++++++++++++++ virt/kvm/kvm_main.c | 39 ++++-- 12 files changed, 243 insertions(+), 60 deletions(-) create mode 100644 tools/testing/selftests/kvm/x86_64/memory_slot_pci_hole.c -- 2.25.4
