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

[Marc Zyngier <marc.zyngier@arm.com>]

On 06/10/17 14:47, Alex Bennée wrote:
> 
> Marc Zyngier <marc.zyngier@arm.com> writes:
> 
>> On 06/10/17 13:37, Marc Zyngier wrote:
>>> On 06/10/17 12:39, Alex Bennée 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 Bennée <alex.bennee@linaro.org>
>>>> ---
>>>>  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);
>>>>
>>>> -	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 */
>>>> +	return kvm_arm_maybe_return_debug(vcpu, run);
>>>
>>> Ah, that's how you do it. OK. Then the patch splitting is wrong, because
>>> everything is broken after patch #1.
>>
>> Actually, it is not broken at all. I'm just confused by the very
>> esoteric flow.
> 
> We could just merge the whole patch in one but I wanted to show the
> difference between in-kernel and out-of-kernel emulation.
> 
> I could also move the step handling to the mmio leg in
> kvm_arch_vcpu_ioctl_run but you mentioned you use the mmio completion
> elsewhere anyway?
Yes, look at the end of io_mem_abort(). This is used by the vgic to
complete a read emulation in the kernel.

And actually, this means that we shouldn't have to mess with
handle_exit. Just check for the return value of kvm_handle_mmio_return
in the call sites (including the one in io_mem_abort), and exit if we
need to single-step...

Thoughts?

	M.
-- 
Jazz is not dead. It just smells funny...