dpw/kvm-hello-world

A minimal kvm example

kvm-hello-world's Issues

/usr/bin/ld: cannot use executable file 'payload.o' as input to a link

Hi,
when the makefile is executed to

 kvm-hello-world: kvm-hello-world.o payload.o
	$(CC) $^ -o $@

a error comes /usr/bin/ld: cannot use executable file 'payload.o' as input to a link

ld version is GNU ld (GNU Binutils) 2.36.1

Are there any good ideas to fix this?
thanks.

hlt instruction qemu-kvm

Hi.

I'm trying to implement that a user program in a guest OS executes an instruction(or function) and qemu receives it.
What you mentioned about vmcall and hlt is really helpful and your hello world example can do what I wanna do exactly.

I can find the ioctl(KVM_RUN) in kvm-all.c in the qemu source tree. I added a test code right after this ioctl and checked that exit_reason is same with KVM_EXIT_HLT.
First, I executed hlt instruction in a user program in a guest machine.
It caused a segfault and qemu received nothing. I thought it is because hlt is a privileged instruction.

So I added a syscall which just executes hlt into a guest kernel, and make a user program execute this syscall.
But qemu's exit reason was never the same as a KVM_EXIT_HLT.

Using perf tool, however, I can see that there are a lot of vm exit caused by the halt when I executed this syscall repeatedly. It seems that vmexit is occured, but because of somewhat reason, it is not delivered to qemu.

It might be trivial problem... but I'm really new to low level things.
What did I miss?

I will be really appreciated if you give me an advice.
Thanks in advance.

kvm-hello-world on AMD

I tried this code on AMD, first on the host machine and also on a virtual machine. It works as expected on the host machine, but it crashed the virtual machine (and QEMU which I used to launch the virtual machine). Do you have any suggestion on how to make it work on a virtual machine?

64 bit mode broken in Ubuntu 18.04 LTS (KVM_SET_SREGS: Invalid argument)

Hi,

since Ubuntu 18.04 LTS came out 2 days ago I performed a clean install of Ubuntu 18.04 LTS 64 bit.
Unfortunately in the latest Ubuntu Version the Long Mode Version seems broken:
ioctl(vcpu->fd, KVM_SET_SREGS, &sregs) (run_long_mode() line 410 kvm-hello-world.c) fails with:

KVM_SET_SREGS: Invalid argument

All other Modes (16 bit, 32 bit, 32 bit paged) still work fine. Also reinstalling the old Version Ubuntu 17.10 64 bit makes the problem disappear, so it must be related to a change in the KVM module in the latest Ubuntu version.

Taking a closer look at the Code shows that 32 bit Paged Mode and 64 bit Mode Settings only differ in 4 Points:

-Segment Setup:
--seg.db=1 in 32 bit (=> default operand size 32 bit), =0 in 64 bit (=> 16 bit Operand Size)
--seg.l=0 in 32 bit, =1 in 64 bit (=1 enables 64 bit instruction encoding)
-Control Register Setup:
--CR4=CR4_PSE in 32 bit (4 MiB Pages), =CR4_PAE (36 bit extended physical address translation in gpt)
--EFER=0 in 32 bit, =EFER_LME in 64 bit (long mode enable)

Experimenting a bit shows that the problem is probably related to the combination of seg.l=1 and EFER=EFER_LME: when both are reset to the 32 bit settings the problem seems to disappear (to be precise it causes another problem resulting in a KVM_EXIT_SHUTDOWN but it makes the error "KVM_SET_SREGS: Invalid argument" disappear and lets the VM start).

Anyway seg.l=1 and EFER=EFER_LME are essential for entering 64 bit mode and can't be skipped of course - Maybe there was a change in Ubuntu 18.04 KVM regarding long mode in virtual machines? At least I didn't read about any such change...

Could you maybe tell me how to fix this problem?

Thanks a lot in advance for your help :)

Kind regards,
Andreas

Undefined Reference to "_GLOBAL_OFFSET_TABLE_" when linking guest32.o

Hi,

first of all thanks a lot for all the code, I'm currently trying to get started setting up a VM with KVM api (ideally 64 bit mode later on) and these examples help me a lot in understanding how to use the KVM api 👍

Unfortunately I get an error message When trying to compile the code with "make run":

---

Original Output:
ld -T guest.ld -m elf_i386 guest32.o -o guest32.img
guest32.o: In Funktion »_start«:
guest.c:(.start+0x6): Warnung: undefinierter Verweis auf »GLOBAL_OFFSET_TABLE«
Makefile:26: die Regel für Ziel „guest32.img“ scheiterte
make: *** [guest32.img] Fehler 1

---

So ld complains about an undefined reference to "GLOBAL_OFFSET_TABLE" at offset ".start+0x6" when trying to link the guest32.o file into guest32.img using the guest.ld linker script.
(Trying to build this on an Ubuntu 17.10 64 bit OS with Intel® Core™ i5-2400 CPU inside).

Anyway 16bit and 64bit version give no errors when compiling and linking, everything works just fine here.

Did I miss anything for 32bit-mode? Unfortunately I don't have an idea how to fix this problem currently ...
Could you maybe tell me what causes this problem and how to fix it?

Thanks a lot in advance for your help :)

Kind regards,
Andreas

No real-mode output?

Notice, no output after "Testing real mode".

$make
cc -Wall -Wextra -Werror -O2 -g   -c -o kvm-hello-world.o kvm-hello-world.c
as   -o guest16.o guest16.s
cc -Wall -Wextra -Werror -O2 -g -m32 -ffreestanding -fno-pic -c -o guest32.o guest.c
ld -T guest.ld -m elf_i386 guest32.o -o guest32.img
ld -b binary -r guest32.img -o guest32.img.o
cc -Wall -Wextra -Werror -O2 -g -m64 -ffreestanding -fno-pic -c -o guest64.o guest.c
ld -T guest.ld guest64.o -o guest64.img
ld -b binary -r guest64.img -o guest64.img.o
ld -T payload.ld -o payload.o
cc kvm-hello-world.o payload.o -o kvm-hello-world
./kvm-hello-world
Testing real mode
./kvm-hello-world -s
Testing protected mode
Hello, world!
./kvm-hello-world -p
Testing 32-bit paging
Hello, world!
./kvm-hello-world -l
Testing 64-bit mode
Hello, world!

Question about -Werror in makefile

On some platforms. The compiler may check the array-bounds and see this line as an error.

cc -Wall -Wextra -Werror -O2 -m32 -ffreestanding -fno-pic -c -o guest32.o guest.c
guest.c: In function ‘_start’:
guest.c:17:9: error: array subscript 0 is outside array bounds of ‘long int[0]’ [-Werror=array-bounds=]
   17 |         *(long *) 0x400 = 42;
      |         ^~~~~~~~~~~~~~~
cc1: note: source object is likely at address zero

Removing the -Werror in makefile will fix it.

--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,4 @@
-CFLAGS = -Wall -Wextra -Werror -O2
+CFLAGS = -Wall -Wextra -O2

Question: where is string "Hello world!" data

0000000000000000 <_start>:
   0:   f3 0f 1e fa             endbr64 
   4:   ba 00 00 00 00          mov    $0x0,%edx
   9:   b8 48 00 00 00          mov    $0x48,%eax
   e:   e6 e9                   out    %al,$0xe9
  10:   0f b6 42 01             movzbl 0x1(%rdx),%eax
  14:   48 83 c2 01             add    $0x1,%rdx
  18:   84 c0                   test   %al,%al
  1a:   75 f2                   jne    e <_start+0xe>
  1c:   48 c7 04 25 00 04 00    movq   $0x2a,0x400
  23:   00 2a 00 00 00 
  28:   b8 2a 00 00 00          mov    $0x2a,%eax
  2d:   0f 1f 00                nopl   (%rax)
  30:   f4                      hlt    
  31:   eb fd                   jmp    30 <_start+0x30>

This is objdump result of guest64.o
It seems that no "Hello world!" string data in this code.
(I think line9 of 0x48 is ASCI code of "H")
Why can this guest output "Hello world!" when it execute on Host VMM(lvm-hello-world)?

Question on KVM

I've not been able to figure out an answer to this, but how many KVM instances can you make on a host machine, is there some limit? Is there a Kernel limit in Linux, or a hardware limit?

Obviously ram would be a limiting factor, but say I want to have 32gb of ram allocated to as many VM's as a 2MB memory limit would allow, is that possible? That would be 16k of VM instances, is that actually doable?

In fact, this is an issue, but a comment.

This code is very very helpful for the one just beginning to learn to KVM, thanks for the excellent job.
I just finished testing the MMIO emulation based on your code (changed your code to support KVM_MMIO_EXIT).

And I would like to see your code to have more and more features to show the features/functionalities of KVM.

And here is another example in github for KVM API for your reference.
https://github.com/hitmoon/kvmsample/blob/master/main.c.

How to run this proyect?

Hello, sorry for this issue but I have a problem with the kvm file.
The problem that I have is that I don't know how to run the file. When I try to run it my Kali says that
"open /dev/kvm: No such file or directory" and I don't know what to do, I followed many tutorials about how to run this kind of binaries but none of these work for me.
Can you help me please?

License file

Thanks a lot, this is a really helpful sample!
Would it be possible to add a LICENSE file, e.g. MIT or Apache 2?