Subject: [PATCH v1 2/2] kvm: arm64: handle single-step of userspace mmio instructions

From: [email-masked] (Christoffer Dall)
Date: Sat, 14 Oct 2017 07:20:05 -0700

On Fri, Oct 13, 2017 at 10:27:36AM +0100, Alex Benn?e wrote:
> 
> Christoffer Dall <[email-masked]> writes:
> 
> > On Fri, Oct 06, 2017 at 12:39:21PM +0100, Alex Benn?e wrote:
> >> The system state of KVM when using userspace emulation is not complete
> >> until we return into KVM_RUN. To handle mmio related updates we wait
> >> until they have been committed and then schedule our KVM_EXIT_DEBUG.
> >>
> >> I've introduced a new function kvm_arm_maybe_return_debug() to wrap up
> >> the differences between arm/arm64 which is currently null for arm.
> >>
> >> Signed-off-by: Alex Benn?e <[email-masked]>
> >> ---
> >>  arch/arm/include/asm/kvm_host.h   |  2 ++
> >>  arch/arm64/include/asm/kvm_host.h |  1 +
> >>  arch/arm64/kvm/debug.c            | 21 +++++++++++++++++++++
> >>  arch/arm64/kvm/handle_exit.c      |  9 +++------
> >>  virt/kvm/arm/arm.c                |  2 +-
> >>  virt/kvm/arm/mmio.c               |  3 ++-
> >>  6 files changed, 30 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> >> index 4a879f6ff13b..aec943f6d123 100644
> >> --- a/arch/arm/include/asm/kvm_host.h
> >> +++ b/arch/arm/include/asm/kvm_host.h
> >> @@ -285,6 +285,8 @@ static inline void kvm_arm_init_debug(void) {}
> >>  static inline void kvm_arm_setup_debug(struct kvm_vcpu *vcpu) {}
> >>  static inline void kvm_arm_clear_debug(struct kvm_vcpu *vcpu) {}
> >>  static inline void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu) {}
> >> +static inline int  kvm_arm_maybe_return_debug(struct kvm_vcpu *vcpu,
> >> +						struct kvm_run *run) {}
> >>
> >>  int kvm_arm_vcpu_arch_set_attr(struct kvm_vcpu *vcpu,
> >>  			       struct kvm_device_attr *attr);
> >> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> >> index e923b58606e2..fa67d21662f6 100644
> >> --- a/arch/arm64/include/asm/kvm_host.h
> >> +++ b/arch/arm64/include/asm/kvm_host.h
> >> @@ -369,6 +369,7 @@ void kvm_arm_init_debug(void);
> >>  void kvm_arm_setup_debug(struct kvm_vcpu *vcpu);
> >>  void kvm_arm_clear_debug(struct kvm_vcpu *vcpu);
> >>  void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu);
> >> +int  kvm_arm_maybe_return_debug(struct kvm_vcpu *vcpu, struct kvm_run *run);
> >>  int kvm_arm_vcpu_arch_set_attr(struct kvm_vcpu *vcpu,
> >>  			       struct kvm_device_attr *attr);
> >>  int kvm_arm_vcpu_arch_get_attr(struct kvm_vcpu *vcpu,
> >> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> >> index dbadfaf850a7..a10a18c55c87 100644
> >> --- a/arch/arm64/kvm/debug.c
> >> +++ b/arch/arm64/kvm/debug.c
> >> @@ -221,3 +221,24 @@ void kvm_arm_clear_debug(struct kvm_vcpu *vcpu)
> >>  		}
> >>  	}
> >>  }
> >> +
> >> +
> >> +/*
> >> + * When KVM has successfully emulated the instruction we might want to
> >> + * return we a KVM_EXIT_DEBUG. We can only do this once the emulation
> >> + * is complete though so for userspace emulations we have to wait
> >> + * until we have re-entered KVM.
> >> + *
> >> + * Return > 0 to return to guest, 0 (and set exit_reason) on proper
> >> + * exit to userspace.
> >> + */
> >> +
> >> +int kvm_arm_maybe_return_debug(struct kvm_vcpu *vcpu, struct kvm_run *run)
> >> +{
> >> +	if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
> >> +		run->exit_reason = KVM_EXIT_DEBUG;
> >> +		run->debug.arch.hsr = ESR_ELx_EC_SOFTSTP_LOW << ESR_ELx_EC_SHIFT;
> >> +		return 0;
> >> +	}
> >> +	return 1;
> >> +}
> >> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> >> index c918d291cb58..7b04f59217bf 100644
> >> --- a/arch/arm64/kvm/handle_exit.c
> >> +++ b/arch/arm64/kvm/handle_exit.c
> >> @@ -202,13 +202,10 @@ static int handle_trap_exceptions(struct kvm_vcpu *vcpu, struct kvm_run *run)
> >>  		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;
> >> -	}
> >> +	if (handled)
> >> +		return kvm_arm_maybe_return_debug(vcpu, run);
> >
> > Again, this seems to override the return value of exit_handler, which
> > may be something negative.
> >
> > Just so I'm clear: There's no intended functionality change of this
> > particular hunk, it's just to share the logic in
> > kvm_arm_maybe_return_debug, right?
> 
> Yes, modulo the annoying semantics in the two places of the vcpu run
> ioctl loop.
> 
> >
> >>
> >> -	return handled;
> >> +	return 0;
> >>  }
> >>
> >>  /*
> >> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> >> index b9f68e4add71..3d28fe2daa26 100644
> >> --- a/virt/kvm/arm/arm.c
> >> +++ b/virt/kvm/arm/arm.c
> >> @@ -623,7 +623,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
> >>
> >>  	if (run->exit_reason == KVM_EXIT_MMIO) {
> >>  		ret = kvm_handle_mmio_return(vcpu, vcpu->run);
> >> -		if (ret)
> >> +		if (ret < 1)
> >>  			return ret;
> >>  	}
> >>
> >> diff --git a/virt/kvm/arm/mmio.c b/virt/kvm/arm/mmio.c
> >> index b6e715fd3c90..e43e3bd6222f 100644
> >> --- a/virt/kvm/arm/mmio.c
> >> +++ b/virt/kvm/arm/mmio.c
> >> @@ -117,7 +117,8 @@ int kvm_handle_mmio_return(struct kvm_vcpu *vcpu, struct kvm_run *run)
> >>  		vcpu_set_reg(vcpu, vcpu->arch.mmio_decode.rt, data);
> >>  	}
> >>
> >> -	return 0;
> >> +	/* If debugging in effect we may need to return now */
> >
> > Will this ever be about other types of debugging (watchpoint on a MMIO
> > access?) or should we limit the text and description to
> > single-stepping?
> 
> Hmm I don't think so. A hbreak should hit (via normal exception path)
> before we attempt any emulation. I suspect watchpoints wouldn't hit for
> emulation though - that would be trickier to do nicely though as it
> would need to be checked for in both kernel and userspace emulation.
> 
> >

Then I think we should be specific in function naming and comments and
refer to single-stepping as opposed to something more generic, because
single-stepping seems to be the case we care about.

Thanks,
-Christoffer