Christoffer Dall <christoffer.dall@xxxxxxxxxx> writes:
> On Thu, Jun 25, 2015 at 07:38:33AM +0100, Alex Bennée wrote:
>>
>> Christoffer Dall <christoffer.dall@xxxxxxxxxx> writes:
>>
>> > On Fri, Jun 19, 2015 at 01:23:48PM +0100, Alex Bennée wrote:
>> >> This adds support for userspace to control the HW debug registers for
>> >> guest debug. In the debug ioctl we copy the IMPDEF defined number of
<snip>
>> >> void kvm_arm_clear_debug(struct kvm_vcpu *vcpu)
>> >> {
>> >> - if (vcpu->guest_debug)
>> >> + if (vcpu->guest_debug) {
>> >> restore_guest_debug_regs(vcpu);
>> >> +
>> >> + /*
>> >> + * If we were using HW debug we need to restore the
>> >> + * debug_ptr to the guest debug state.
>> >> + */
>> >> + if (vcpu->guest_debug & KVM_GUESTDBG_USE_HW)
>> >> + vcpu->arch.debug_ptr = &vcpu->arch.vcpu_debug_state;
>> >
>> > I still think this would be more cleanly done in the setup_debug
>> > function, but ok:
>>
>> I don't follow, setup_debug is called before we enter KVM. It's pretty
>> light when no debugging is being done so this ensure we leave state how
>> we would like it when we stop debugging.
>>
>> I can move it to an else leg in setup if you really want.
>>
> I just feel like whenever you enter the guest you setup the state you
> want for your guest and then when reading the code you never have to
> worry about "did I set the pointer back correctly last time it exited",
> but thinking about your response, I guess that's an extra store on each
> world-switch, so theoretically that may be a bit more overhead (on top
> of the hundreds other stores and spinlocks we take and stuff).
The setup/clear() calls are tightly paired around the KVM_RUN ioctl code
without any obvious exit points.
Are there any cases you can escape the ioctl code flow? I notice irq's
are re-enabled so I guess a suitably determined irq function could
change the return address or mess around with guest_debug.
> If you prefer, leave it like this, but consider adding a
> BUG_ON(!guest_debugging && debug_ptr != &vcpu->arch.vcpu_debug_state) in
> the setup function...
The clear_debug() code would end up being a fairly sparse piece of code
without it ;-)
> I'm probably being paranoid.
A little paranoia goes a long way in kernel mode ;-)
>
> -Christoffer
--
Alex Bennée
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