Re: [PATCH v1 1/2] KVM: arm64: handle single-stepping trapped instructions

[Julien Thierry]

On 06/10/17 13:27, Marc Zyngier wrote:
> On 06/10/17 12:39, Alex BennÃe wrote:
> > If we are using guest debug to single-step the guest we need to ensure
> > we exit after emulating the instruction. This only affects
> > instructions completely emulated by the kernel. For userspace emulated
> > instructions we need to exit and return to complete the emulation.
> >
> > We fake debug.arch.hsr to contain ESR_ELx_EC_SOFTSTP_LOW so QEMU knows
> > it was a single-step event (and without altering the userspace ABI).
> >
> > Signed-off-by: Alex BennÃe <alex.bennee@xxxxxxxxxx>
> > ---
> >   arch/arm64/kvm/handle_exit.c | 48 +++++++++++++++++++++++++++++++-------------
> >   1 file changed, 34 insertions(+), 14 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> > index 7debb74843a0..c918d291cb58 100644
> > --- a/arch/arm64/kvm/handle_exit.c
> > +++ b/arch/arm64/kvm/handle_exit.c
> > @@ -178,6 +178,39 @@ static exit_handle_fn kvm_get_exit_handler(struct kvm_vcpu *vcpu)
> >   	return arm_exit_handlers[hsr_ec];
> >   }
> >   
> > +/*
> > + * When handling traps we need to ensure exit the guest if we
> > + * completely emulated the instruction while single-stepping. Stuff to
> > + * be emulated in userspace needs to complete that first.
> > + */
> > +
> > +static int handle_trap_exceptions(struct kvm_vcpu *vcpu, struct kvm_run *run)
> > +{
> > +	int handled;
> > +
> > +	/*
> > +	 * See ARM ARM B1.14.1: "Hyp traps on instructions
> > +	 * that fail their condition code check"
> > +	 */
> > +	if (!kvm_condition_valid(vcpu)) {
> > +		kvm_skip_instr(vcpu, kvm_vcpu_trap_il_is32bit(vcpu));
> > +		handled = 1;
> > +	} else {
> > +		exit_handle_fn exit_handler;
> > +
> > +		exit_handler = kvm_get_exit_handler(vcpu);
> > +		handled = exit_handler(vcpu, run);
> > +	}
> > +
> > +	if (handled && (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)) {
> > +		handled = 0;
> > +		run->exit_reason = KVM_EXIT_DEBUG;
> > +		run->debug.arch.hsr = ESR_ELx_EC_SOFTSTP_LOW << ESR_ELx_EC_SHIFT;
> > +	}
>
> Doesn't this break an MMIO read? The registers haven't been updated yet,
> and the debugger may not see the right thing...
>
> How about something like:
>
> 	if (handled && (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)) {
> 		if (run->exit_reason == KVM_EXIT_MMIO)
> 			kvm_handle_mmio_return(vcpu, run);
> 		[...]
> 	}
>
> Or am I missing something?

If the MMIO was not handled by the kernel, exit_handler will return 0, 
so handled will be false and we won't pretend we have a debug exception 
(but will still return to userland with KVM_EXIT_MMIO).

I think the second patch takes care of properly handling single step for 
userland MMIO.

--
Julien Thierry