[转] KVM Internals, code and more

KVM

Kernel-based Virtual Machine

Internals, code and more http://slides.com/braoru/kvm#/

What behind KVM QEMU and KVM architecture overview KVM internals Very small Introduction to Libvirt

KVM in 5 secondes

Introduced to make VT-x/AMD-V available to user space Exposes virtualization features securely through a single interface /dev/kvm vailable since 2.6.20 (2006) Clean and efficient dev From first LKML posting to merge: 3 months 100% orthogonal to core kernel

KVM is not KVM

First of all there is QEMU then KVM then Libvirt then the whole ecosystems..

At the begining, Qemu

Running a guest involves executing guest code Handling timers Processing I/O Responding to monitor commands. Doing all these things at once without pausing guest execution
Deal with events

Deal with events

There are two popular architectures for programs that need to respond to events from multiple sources

DEAL WITH EVENTS

Parallel architecture

Splits work into processes or threads that can execute simultaneously.

DEAL WITH EVENTS

Event-driven architecture

Event-driven architecture reacts to events by running a main loop that dispatches to event handlers. This is commonly implemented using the select(2) or poll(2) family of system calls to wait on multiple file descriptors.

Threading and event driven model of qemu

Qemu uses an hybrid architecture

Qemu the event_loop

Event-driven architecture is centered around the event loop which dispatches events to handler functions. QEMU's main event loop is main_loop_wait() Waits for file descriptors to become readable or writable. File descriptors are a critical because files, sockets, pipes, and various other resources are all file descriptors. Runs expired timers. Runs bottom-halves (BHs), which used to avoid reentrancy and overflowing the call stack.
main-loop.c

QEMU THE EVENT_LOOP

A file descriptor becomes ready, a timer expires, or a BH is scheduled, the event loop invokes a callback No other core code is executing at the same time so synchronization is not necessary Execute sequentially and atomically Only 1 thread of control needed at any given time No blocking system calls or long-running computations should be performed. Avoid spending an unbounded amount of time in a callback If you not follow those advices this will force the guest to pause and the monitor to become unresponsive.

QEMU threads

To help the event_loop Offload what need to be offloaded

QEMU THREADS

TO HELP THE EVENT_LOOP There are system calls which have no non-blocking equivalent. Sometimes long-running computations flood the CPU and can't be easily break up into callbacks. In these cases dedicated worker threads can be used to carefully move these tasks out of core QEMU. One example of worker threads is vnc-jobs.c When a worker thread needs to notify core QEMU, a pipe or a qemu_eventfd() file descriptor is added to the event loop.

executing guest code

Here are two mechanism for executing guest code: Tiny Code Generator (TCG) and KVM Executing guest code in qemu is very simple, it use thread. Exactly 1 thread by vcpu.

summary about qemu processing

1 process per guest 1 thread for the main event_loop() 1 thread by vcpu As many (reasonably) threads as needed for offloaded tasks
All the existing Linux strengths at our disposal Memory as Huge page, KSM, IO, Scheduler, Energy, Device hotplug, networking, Security, All the Linux software world, ...

qemu guest memory

Guest ram is allocated at qemu start up This mapped memory is "really" allocated by the process (with malloc()) qemu_madvise() Tips : use -mem-path to give qemu a memory image to load (can be very very good in #infosec)

reminder

why x86 virt is a pain... No hardware provisions Instruction behave differently depending on privilege context Architecture not built for trap and emulate CISC is ... CISC
A complete theorical virtualisation courses : CS 686: Special Topic: Intel EM64T and VT Extensions (Spring 2007)

reminder

how intel vt-x help Guest SW <-> VMM Transitions Virtual-machine control structure

KVM virtualisation

KVM is a virtualization feature in the Linux kernel that lets you safely execute guest code directly on the host CPU open /dev/kvm use iocrl KVM_RUN (KVM IOCTL doc)
As simple as :

open("/dev/kvm")
ioctl(KVM_CREATE_VM)
ioctl(KVM_CREATE_VCPU)
for (;;) {
ioctl(KVM_RUN)
switch (exit_reason) {
case KVM_EXIT_IO:  /* ... */
case KVM_EXIT_HLT: /* ... */
}
}

kvm virtualisation

It's DEMO time
What do you need :
A bit of C ...
A touch of ASM
Makefile
gcc

EMU / KVM / CPU / TIME interactions

Light vs Heavy exit

QEMU / KVM / CPU / TIME interactions

causes of VM Exits

VM Entry :
Transition from VMM to Guest Enters VMX non-root operation
Loads Guest state and Exit criteria from VMCS
VMLAUNCH instruction used on initial entry
VMRESUME instruction used on subsequent entries

VM Exit :
VMEXIT instruction used on transition from Guest to VMM
Enters VMX root operation
Saves Guest state in VMCS
Loads VMM state from VMCS

start a kvm vm in reality

A bit more complicated than before :

KVM CREATE VM : The new VM has no virtual cpus and no memory
KVM SET USER MEMORY REGION : MAP userspace memory for the VM
KVM CREATE IRQCHIP / ...PIT KVM CREATE VCPU : Create hardware component and map them with VT-X functionnalities
KVM SET REGS / ...SREGS / ...FPU / ... KVM SET CPUID / ...MSRS / ...VCPU EVENTS / ... KVM SET LAPIC : hardware configurations
KVM RUN : Start the VM

start a vm in qemu-kvm

/usr/bin/qemu-kvm -S -M pc-0.13 -enable-kvm -m 512 -smp 2,sockets=2,cores=1,threads=1
-name test -uuid e9b4c7be-d60a-c16e-92c3-166421b4daca -nodefconfig -nodefaults
-chardev socket,id=monitor,path=/var/lib/libvirt/qemu/test.monitor,server,nowait
-mon chardev=monitor,mode=readline -rtc base=utc -boot c
-drive file=/var/lib/libvirt/images/test.img,if=none,id=drive-virtio-disk0,boot=on,format=raw
-device virtio-blk-pci,bus=pci.0,addr=0x5,drive=drive-virtio-disk0,id=virtio-disk0
-drive if=none,media=cdrom,id=drive-ide0-1-0,readonly=on,format=raw
-device ide-drive,bus=ide.1,unit=0,drive=drive-ide0-1-0,id=ide0-1-0
-device virtio-net-pci,vlan=0,id=net0,mac=52:54:00:cc:1c:10,bus=pci.0,addr=0x3
-net tap,fd=59,vlan=0,name=hostnet0 -chardev pty,id=serial0 -device isa-serial,chardev=serial0
-usb -device usb-tablet,id=input0 -vnc 127.0.0.1:0 -vga cirrus -device AC97,id=sound0,bus=pci.0,addr=0x4
-device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x6

Now you really know why tools are great

KVM processing

What about passthroug, paravirt and virtio

Reduce VM exits or make them lightweight
Improve I/O throughput & latency (less emulation)
Compensates virtualization effects
Enable direct host-guest interaction

VIRTIO device

Network
Block
Serial I/O (console, host-guest channel, ...)
Memory balloon
File system (9P)
SCSI

Based on generic RX/TX buffer
Logic distributed in the guest driver (aka virtual device) and qemu backend (and kernel backend in some cases)

Virtio device

vhost example

High throughput
Low latency guest networking

Normally the QEMU userspace process emulates I/O accesses from the guest. Vhost puts virtio emulation code into the kernel

Dont forget vhost-blk and vhost-scsi

Virtio

vhost example

vhost-net driver creates a /dev/vhost-net character device on the host
QEMU is launched with -netdev tap,vhost=on and open /dev/vhost-net
vhost driver creates a kernel thread called vhost-$pid
$pid = pidof(QEMU)
Job of the worker thread is to handle I/O events and perform the device emulation.
vhost architecture is not directly linked to KVM
Use ioeventfd and irqfd

Virtio device

Virtio device

vhost
Kernel code :
drivers/vhost/vhost.c - common vhost driver code
drivers/vhost/net.c - vhost-net driver
virt/kvm/eventfd.c - ioeventfd and irqfd

The QEMU userspace code shows how to initialize the vhost instance :

hw/vhost.c - common vhost initialization code
hw/vhost_net.c - vhost-net initialization

LIBVIRT

very small introduction

Virtualization library: manage guest on one or many nodes
Share the application stack between hypervisors
Long term stability and compatibility of API and ABI
Provide security and remote access “out of the box”
Expand to management APIs (Node, Storage, Network)

livirt

Very small introduction