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

["Alex Bennée" <alex.bennee@linaro.org>]

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?

--
Alex Bennée
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