Kernel debugging with KVM?

Kernel debugging with KVM?

[Jun Koi]

Hi,

I am investigating the possibility of debugging guest kernel with KVM.
Anybody may shed some lights into how KVM handles breakpoints?

Suppose that we put a breakpoint (0xCC opcode in x86) into guest
kernel. When the kernel hits the breakpoint, which one will handle the
event?

Looks like qemu will handle the breakpoint first, then next the guest
kernel itself have a chance to process it. Is that correct?

And is there any chance for the host (VMM) to get notified about the event?

Thank you,
Jun

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Re: Kernel debugging with KVM?

[Avi Kivity]

Jun Koi wrote:
> Hi,
>
> I am investigating the possibility of debugging guest kernel with KVM.
> Anybody may shed some lights into how KVM handles breakpoints?
>
> Suppose that we put a breakpoint (0xCC opcode in x86) into guest
> kernel. When the kernel hits the breakpoint, which one will handle the
> event?
>
> Looks like qemu will handle the breakpoint first, then next the guest
> kernel itself have a chance to process it. Is that correct?
>
> And is there any chance for the host (VMM) to get notified about the event?
>
>   

kvm supports using hardware breakpoints to debug a guest.  See the 
KVM_GUEST_DEBUG ioctl.

The modified qemu shipped with kvm allows gdb to connect and debug a kvm 
guest.  It's been a long while since I tested this, so it may or may not 
work.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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