Running virtual machines with qemu-system-*

In this recipe, we are going to demonstrate how to start virtual machines with QEMU. QEMU provides binaries that can emulate different CPU architectures using either custom or prebuilt images for the guest OS.

If you completed the Installing and configuring QEMU recipe, you should have a host that contains the following binaries:

root@kvm:~# ls -la /usr/bin/qemu-system-*
-rwxr-xr-x 1 root root 8868848 Jan 25 12:49 /usr/bin/qemu-system-aarch64
-rwxr-xr-x 1 root root 7020544 Jan 25 12:49 /usr/bin/qemu-system-alpha
-rwxr-xr-x 1 root root 8700784 Jan 25 12:49 /usr/bin/qemu-system-arm
-rwxr-xr-x 1 root root 3671488 Jan 25 12:49 /usr/bin/qemu-system-cris
-rwxr-xr-x 1 root root 8363680 Jan 25 12:49 /usr/bin/qemu-system-i386
-rwxr-xr-x 1 root root 3636640 Jan 25 12:49 /usr/bin/qemu-system-lm32
-rwxr-xr-x 1 root root 6982528 Jan 25 12:49 /usr/bin/qemu-system-m68k
-rwxr-xr-x 1 root root 3652224 Jan 25 12:49 /usr/bin/qemu-system-microblaze
-rwxr-xr-x 1 root root 3652224 Jan 25 12:49 /usr/bin/qemu-system-microblazeel
-rwxr-xr-x 1 root root 8132992 Jan 25 12:49 /usr/bin/qemu-system-mips
-rwxr-xr-x 1 root root 8356672 Jan 25 12:49 /usr/bin/qemu-system-mips64
-rwxr-xr-x 1 root root 8374336 Jan 25 12:49 /usr/bin/qemu-system-mips64el
-rwxr-xr-x 1 root root 8128896 Jan 25 12:49 /usr/bin/qemu-system-mipsel
-rwxr-xr-x 1 root root 3578592 Jan 25 12:49 /usr/bin/qemu-system-moxie
-rwxr-xr-x 1 root root 3570848 Jan 25 12:49 /usr/bin/qemu-system-or32
-rwxr-xr-x 1 root root 8701760 Jan 25 12:49 /usr/bin/qemu-system-ppc
-rwxr-xr-x 1 root root 9048000 Jan 25 12:49 /usr/bin/qemu-system-ppc64
lrwxrwxrwx 1 root root 17 Jan 25 12:49 /usr/bin/qemu-system-ppc64le -> qemu-system-ppc64
-rwxr-xr-x 1 root root 8463680 Jan 25 12:49 /usr/bin/qemu-system-ppcemb
-rwxr-xr-x 1 root root 6894528 Jan 25 12:49 /usr/bin/qemu-system-sh4
-rwxr-xr-x 1 root root 6898624 Jan 25 12:49 /usr/bin/qemu-system-sh4eb
-rwxr-xr-x 1 root root 4032000 Jan 25 12:49 /usr/bin/qemu-system-sparc
-rwxr-xr-x 1 root root 7201696 Jan 25 12:49 /usr/bin/qemu-system-sparc64
-rwxr-xr-x 1 root root 3704704 Jan 25 12:49 /usr/bin/qemu-system-tricore
-rwxr-xr-x 1 root root 3554912 Jan 25 12:49 /usr/bin/qemu-system-unicore32
-rwxr-xr-x 1 root root 8418656 Jan 25 12:49 /usr/bin/qemu-system-x86_64
-rwxr-xr-x 1 root root 3653024 Jan 25 12:49 /usr/bin/qemu-system-xtensa
-rwxr-xr-x 1 root root 3642752 Jan 25 12:49 /usr/bin/qemu-system-xtensaeb
root@kvm:~#

Each command can start a QEMU-emulated instance for the specific CPU architecture. For this recipe, we are going to be using the qemu-system-x86_64 utility.

Getting ready

To complete this recipe, you will need the following:

• The QEMU binaries, provided after following the Installing and configuring QEMU recipe
• The custom raw Debian image we built in the Installing a custom OS on the image with debootstrap recipe
• The CentOS qcow2 image we downloaded in the Using pre-existing images recipe

Let's have a look at what CPU architectures QEMU supports on the host system:

root@kvm:~# qemu-system-x86_64 --cpu help
x86 qemu64 QEMU Virtual CPU version 2.5+
x86 phenom AMD Phenom(tm) 9550 Quad-Core Processor
x86 core2duo Intel(R) Core(TM)2 Duo CPU T7700 @ 2.40GHz
x86 kvm64 Common KVM processor
x86 qemu32 QEMU Virtual CPU version 2.5+
x86 kvm32 Common 32-bit KVM processor
x86 coreduo Genuine Intel(R) CPU T2600 @ 2.16GHz
x86 486
x86 pentium
x86 pentium2
x86 pentium3
x86 athlon QEMU Virtual CPU version 2.5+
x86 n270 Intel(R) Atom(TM) CPU N270 @ 1.60GHz
x86 Conroe Intel Celeron_4x0 (Conroe/Merom Class Core 2)
x86 Penryn Intel Core 2 Duo P9xxx (Penryn Class Core 2)
x86 Nehalem Intel Core i7 9xx (Nehalem Class Core i7)
x86 Westmere Westmere E56xx/L56xx/X56xx (Nehalem-C)
x86 SandyBridge Intel Xeon E312xx (Sandy Bridge)
x86 IvyBridge Intel Xeon E3-12xx v2 (Ivy Bridge)
x86 Haswell-noTSX Intel Core Processor (Haswell, no TSX)
x86 Haswell Intel Core Processor (Haswell)
x86 Broadwell-noTSX Intel Core Processor (Broadwell, no TSX)
x86 Broadwell Intel Core Processor (Broadwell)
x86 Opteron_G1 AMD Opteron 240 (Gen 1 Class Opteron)
x86 Opteron_G2 AMD Opteron 22xx (Gen 2 Class Opteron)
x86 Opteron_G3 AMD Opteron 23xx (Gen 3 Class Opteron)
x86 Opteron_G4 AMD Opteron 62xx class CPU
x86 Opteron_G5 AMD Opteron 63xx class CPU
x86 host KVM processor with all supported host features (only available in KVM mode)

Recognized CPUID flags:
fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 pn clflush ds acpi mmx fxsr sse sse2 ss ht tm ia64 pbe
pni|sse3 pclmulqdq|pclmuldq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cid fma cx16 xtpr pdcm pcid dca sse4.1|sse4_1 sse4.2|sse4_2 x2apic movbe popcnt tsc-deadline aes xsave osxsave avx f16c rdrand hypervisor
fsgsbase tsc_adjust bmi1 hle avx2 smep bmi2 erms invpcid rtm mpx avx512f rdseed adx smap pcommit clflushopt clwb avx512pf avx512er avx512cd
syscall nx|xd mmxext fxsr_opt|ffxsr pdpe1gb rdtscp lm|i64 3dnowext 3dnow
lahf_lm cmp_legacy svm extapic cr8legacy abm sse4a misalignsse 3dnowprefetch osvw ibs xop skinit wdt lwp fma4 tce nodeid_msr tbm topoext perfctr_core perfctr_nb
invtsc
xstore xstore-en xcrypt xcrypt-en ace2 ace2-en phe phe-en pmm pmm-en
kvmclock kvm_nopiodelay kvm_mmu kvmclock kvm_asyncpf kvm_steal_time kvm_pv_eoi kvm_pv_unhalt kvmclock-stable-bit
npt lbrv svm_lock nrip_save tsc_scale vmcb_clean flushbyasid decodeassists pause_filter pfthreshold
xsaveopt xsavec xgetbv1 xsaves
arat
root@kvm:~#

From the preceding output, we can see the list of CPUs that we can pass as parameters to the -cpu flag in order to emulate that CPU type inside our virtual machine.

How to do it...

To start a new virtual machine using the qemu-system utility, perform the following steps:

1. Start a new QEMU virtual machine using the x86_64 CPU architecture:

root@kvm:~# qemu-system-x86_64 -name debian -vnc 146.20.141.254:0 -cpu Nehalem -m 1024 -drive     format=raw,index=2,file=debian.img -daemonize    
root@kvm:~#

2. Ensure that the instance is running:

root@kvm:~# pgrep -lfa qemu
3527 qemu-system-x86_64 -name debian -vnc 146.20.141.254:0 -m 1024 -drive format=raw,index=2,file=debian.img -daemonize    
root@kvm:~#

3. Terminate the Debian QEMU instance:

root@kvm:~# pkill qemu    
root@kvm:~#

4. Start a new QEMU instance using the prebuilt CentOS image:

root@kvm:~# qemu-system-x86_64 -vnc 146.20.141.254:0 -m 1024 -hda CentOS-7-x86_64-GenericCloud.qcow2 -daemonize    
root@kvm:~#

5. Ensure that the instance is running:

root@kvm:~# pgrep -lfa qemu
3546 qemu-system-x86_64 -vnc 146.20.141.254:0 -m 1024 -hda CentOS-7-x86_64-GenericCloud.qcow2 -daemonize    
root@kvm:~#

6. Terminate the CentOS QEMU instance:

root@kvm:~# pkill qemu    
root@kvm:~#

Make sure to replace the IP address of the -vnc parameter with the one from your host machine.

How it works...

How to start a virtual machine with QEMU/KVM depends greatly on the type of image and how the partitions are structured inside that image.

We used two different image types with different partitioning schemes to demonstrate this concept.

In step 1, we used the qemu-system-x86_64 command to emulate a x86_64 CPU architecture, specifically we passed the -cpu Nehalem flag, emulating the Nehalem CPU model. We passed the IP address of our host as a parameter to the -vnc flag. This starts a VNC server in the VM so that we can later use a VNC client to connect to the QEMU instance. We specified the amount of memory to be allocated to the instance, in this case, 1GB with the -m flag. We instructed QEMU that we are going to use a raw image with the format=raw option and the name and location of the actual image with the file=debian.img parameter.

Recall that this raw image contains two partitions with the second partition containing the root filesystem where the bootloader is located. This is very important to remember because we need to specify from what partition index the guest OS should load. We do that with the index=2 flag. Finally, we pass the -daemonize parameter to background the QEMU process.

In step 4, we started another QEMU instance, this time using the qcow2 CentOS image we downloaded earlier. We did not have to specify from what partition we need to boot from this this time because most prebuilt images use the first partition, or only have one partition. We also used the -hda flag instead of the -drive parameter, just to demonstrate that both options can be used with the same result. The -hda flag tells QEMU the first disk for the instance should be loaded from the filename that follows it.