Re: Exposing host debug capabilities to userspace

[Alexander Graf <agraf@suse.de>]

On 25.11.14 17:21, Paolo Bonzini wrote:
> 
> 
> On 24/11/2014 14:59, Alex Bennée wrote:
>> Alexander Graf pointed out that KVM_CHECK_EXTENSION can return any
>> positive number for success. How about using:
>>
>> max_hw_bps = kvm_check_extension(kvm_state, KVM_CAP_GUEST_DEBUG_HW_BPS);
>> max_hw_wps = kvm_check_extension(kvm_state, KVM_CAP_GUEST_DEBUG_HW_WPS);
>>
>> Seems pretty sane, doesn't change the semantics of an API and is
>> architecture agnostic if others need the number?
> 
> Yes, this was going to be my suggestion as well.  Just I would use a
> bitmask in case some register can act as both breakpoint and watchpoint.
> 
> On x86, each of the four bp/wp registers (each register can act as both)
> can be used for either guest or gdbstub debugging.  If the
> KVM_GUESTDBG_USE_HW_BP feature is enabled, the guest is entered with
> "made up" debug register contents, that we pass via
> KVM_SET_GUEST_DEBUG's struct kvm_guest_debug_arch.  Otherwise, the guest
> is entered with real debug register contents passed via
> KVM_SET_DEBUGREGS.  Reads/writes of the debug registers trap to KVM
> (which helps the guest see the expected values of the debug registers in
> the former case).  There is no KVM_GET_GUEST_DEBUG because the
> corresponding info is passed via struct kvm_debug_exit_arch.
> 
> If gdbstub hardware breakpoints are enabled, all hardware breakpoints
> exit to userspace.  QEMU then decides whether the breakpoint came from
> guest debugging (and then injects an exception), or from gdbstub
> debugging (and then suspends execution).  Same for software breakpoints.
>  If the total request is >4 hardware breakpoints, someone will have to
> lose and some gdbstub breakpoints will be missed.

Unfortunately on ARM you also have a few other constraints - the debug
register space is partitioned into magic super debug registers at the
top (with an implementation specific amount) and normal debug registers
in the lower end of the space.

The main pain I have with exposing host information is that it's going
to be interesting and challenging enough to get all of this right merely
for the guest debug register space. Exposing the host debug register
space as well means there is even more space for breakage.

I would just treat gdbstub debugging as the ugly step child that it is
and not introduce anything special for it (except for debug event
deflection to user space). Then only ever work on guest debug registers
and call it a day. Chances are just too high that we get the interfaces
wrong and shoot ourselves in the foot.


Alex