Re: [RFC] Reworking KVM_DEBUG_GUEST

[Hollis Blanchard <hollisb@us.ibm.com>]

On Wednesday 14 May 2008 14:49:02 Jan Kiszka wrote:
> > In Qemu, when exit_reason == KVM_EXIT_DEBUG, it would 
> > just need to see if that address is for a breakpoint Qemu set or not. If 
so, 
> > it's happy. If not, (commence handwaving) tell KVM to forward the debug 
> > interrupt to the guest. This way, the list of breakpoints is maintained in 
> > userspace (in the qemu gdb stub), which is nice because it could be 
> > arbitrarily large.
> 
> Yes, but I would rather pass the debug registers (more general: some
> arch dependent state set) back in this slot. Those contain everything
> the gdbstub needs to know to catch relevant hardware-BP/watchpoint
> events (and report them to the gdb frontend).

But what would the stub *do* with the contents of the debug registers? The 
only reason they were set is on behalf of the stub in the first place. In 
fact, in the case of soft breakpoints, KVM doesn't even know where all the 
set breakpoints are. The only thing KVM needs to report is the address of the 
breakpoint that was just hit.

Sorry if this gets formatted badly:

gdb					qemu stub				KVM
break *0xf00
sends Z0 packet 0xf00
					0xf00 -> BP list
					ioctl(KVM_DEBUG, 0xf00)
continue
					ioctl(KVM_RUN)
											running...
											breakpoint hit
											exit_reason = KVM_EXIT_DEBUG
											kvm_run.debug.address = current PC value
					search BP list for address
bp hit			<---	present        not present --->	send debug interrupt to guest

Notes:
- KVM_DEBUG in this case will set a hardware breakpoint. The alternative is to 
write an int3 into guest memory.
- The stub doesn't care how the hardware registers were configured. All it 
needs to know is a) that a breakpoint was hit, and b) at what address.

Does this make sense?

-- 
Hollis Blanchard
IBM Linux Technology Center

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