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Copyright © 2007 Qumranet, Inc. All rights reserved.
Virtualisation Strategies
“Native” HypervisorsHave a runtimeNeed a “primary” guest OSExamples: Xen, VMWare ESX Server, IBM mainframes
ContainersDifferent namespaces for different guestsRun on host kernelUserland can be different from hostExamples: OpenVZ, FreeVPS, Linux-Vserver
Paravirtualisation
EmulationExamples: QEMU, PearPC
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KVM: Architectures Supported
S390IBM mainframes: a hypervisor is a mustIncluded in 2.6.26
IA-64Included in 2.6.26
X86Included in 2.6.20KVM-lite: PV Linux guest on non-VTx / non-SVM host (proposed)
PowerPCPVArchitecture support for hypervisorIncluded in 2.6.26
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X86 Hardware Extensions
'guest mode' in addition to user and kernel modes
Raise a trap for all privileged instructions
Virtualised registers
ProcessorIntel-VTx (VMX)AMD-V (SVM)
MMEPT (Intel)NPT (AMD)
IOVT-d (Intel)IOMMU (AMD)
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What's handled in the kernel?
CPU virtualisation (special instructions)MMU virtualisationLocal APIC, PIC, IOAPIC, PIT(guest) paravirtualised network and block device drivers
virtio-netvirtio-block
(guest) paravirtualised kernel support codeparavirt_opsMMU
(guest) paravirtualised clock driver
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KVM Process Model
kernel
task task guest task task guest
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KVM Process Model (cont'd)
Guests are scheduled as regular processeskill(1), top(1) work as expectedGuest physical memory is mapped into the task's virtual memory spaceVirtual processors in one VM are threads
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KVM Execution Model
Native GuestExecution
KernelExit Handler
UserspaceExit Handler
Switch toGuest Mode
ioctl()
Userspace Kernel Guest
Lightweight ExitHeavyweight Exit
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Flow Example: MemoryAccess
Guest accesses an unmapped memory locationHardware traps into kernel modekvm walks the guest page table, determines guest physical addresskvm performs guest physical -> host physical translationkvm installs shadow page table entry containing guest virtual -> host physical translationProcessor restarts execution of faulting instruction
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Paravirtualisation
Modifying guest OS for performanceVirtio
Common drivers for all hypervisorsHypervisor-specific backendKVM backend in qemuFaster performanceEfficient block, net driversBalloonlguest, KVM use it already
PV DMAPass through Ethernet devices
paravirt_ops
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Network Devices
Fully virtualised device performance not great55 Mbps for RTLLots of IO-exits per packet
Decided to implement a modern e1000Advantages:
All code in userspace (qemu) All existing drivers recognise device
IRQ coalescingOnly 2-3 IO-exits per packetGoes in excess of 800 Mbps
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Virtio Net
Shared memory between host and guestTwo queues: recv and sendRing buffer within each queue'available' pointer controlled by guest'used' pointer controlled by host
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Virtio-net on KVM
Guest kernel
BLK NET
Virtio PCI
QEMU
Linux
SharedMemory
User-space
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Ideas
Shared memory between host and guest via virtio-pciShared directory between host and guest using virtio + fuseVMGL (OpenGL for Virtual Machines) supporthttp://kvm.qumranet.com/kvmwiki/TODO
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KVM Pros
Leverages Linux scheduler, memory management, I/ONo scheduler involvement for I/OFull virtualisation: No changes to the guest necessary
Paravirt drivers available for better performance
Uses existing Linux security modelcan run VM as ordinary user
Uses existing management toolsPower managementGuest memory swappingReal-time scheduling, NUMALeverages Linux development momentum: all new drivers, {cpu, disk} schedulers, file systems, etc supported
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Distro / Industry interest
libvirtManaging various guests under a hypervisorSupport for Xen, KVMAPIs between UI, middle layer and virtualisation backend
DistributionsDebianUbuntuRedHat ELSLES
QumranetDekstop Virtualisation
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Release Philosophy
Development snapshots every 1-2 weeksRelease early and oftenFeatures introduces quicklyBugs fixed quicklyBugs added quicklyAllows developers and users to track and test the latest and greatest
Stable releases part of Linux 2.6.xWith bugfixes going into Linux 2.6.x.y
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Journey
Linux 2.6.20 (4 Feb 2007): Initial releaseLinux 2.6.21 (25 Apr 2007): Stability, suspend/resumeLinux 2.6.22 (8 Jul 2007): Stable ABI
Old userspace, new kernelNew userspace, old kernel
Linux 2.6.23 (9 Oct 2007): SMP, performanceLinux 2.6.24 (24 Jan 2008): In-kernel APIC, preemptibility, virtioLinux 2.6.25 (16 Apr 2008): Guest swapping, paravirt_ops, balloon drvLinux 2.6.26 (soon): PowerPC, s390, IA64, NPT, EPT, more paravirt (mmu), ...
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KVM is Developer-friendly
No need to reboot (usually)Netconsole, oprofile, all the tools workSmall codebaseFriendly community
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Future
Consolidate various virtualisation solutions existing in the kernel
Started with move to virt/ from drivers/kvm/
More hardware features supportMore paravirtualisation supportImprove guest scalingBetter support for management layers like libvirtIntel Real Mode Emulation
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Do Read
virt/*, arch/[x86|ia64|s390|powerpc]/kvm/*KvmForum2007 wiki page on http://[email protected]@lists.osdl.org
Thank You
