Re: [PATCH v3 06/12] KVM: arm64: guest debug, add SW break point support

[Christoffer Dall <christoffer.dall@linaro.org>]

On Wed, May 06, 2015 at 05:23:21PM +0100, Alex Bennée wrote:
> This adds support for SW breakpoints inserted by userspace.
> 
> We do this by trapping all guest software debug exceptions to the
> hypervisor (MDCR_EL2.TDE). The exit handler sets an exit reason of
> KVM_EXIT_DEBUG with the kvm_debug_exit_arch structure holding the
> exception syndrome information.
> 
> It will be up to userspace to extract the PC (via GET_ONE_REG) and
> determine if the debug event was for a breakpoint it inserted. If not
> userspace will need to re-inject the correct exception restart the
> hypervisor to deliver the debug exception to the guest.
> 
> Any other guest software debug exception (e.g. single step or HW
> assisted breakpoints) will cause an error and the VM to be killed. This
> is addressed by later patches which add support for the other debug
> types.
> 
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> 
> ---
> v2
>   - update to use new exit struct
>   - tweak for C setup
>   - do our setup in debug_setup/clear code
>   - fixed up comments
> v3:
>   - fix spacing in KVM_GUESTDBG_VALID_MASK
>   - fix and clarify wording on kvm_handle_guest_debug
>   - handle error case in kvm_handle_guest_debug
>   - re-word the commit message
> 
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index 4b0132f..5ef937c 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -2667,7 +2667,7 @@ when running. Common control bits are:
>  The top 16 bits of the control field are architecture specific control
>  flags which can include the following:
>  
> -  - KVM_GUESTDBG_USE_SW_BP:     using software breakpoints [x86]
> +  - KVM_GUESTDBG_USE_SW_BP:     using software breakpoints [x86, arm64]
>    - KVM_GUESTDBG_USE_HW_BP:     using hardware breakpoints [x86, s390]
>    - KVM_GUESTDBG_INJECT_DB:     inject DB type exception [x86]
>    - KVM_GUESTDBG_INJECT_BP:     inject BP type exception [x86]
> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> index 4a274e1..064c105 100644
> --- a/arch/arm/kvm/arm.c
> +++ b/arch/arm/kvm/arm.c
> @@ -302,7 +302,7 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>  	kvm_arm_set_running_vcpu(NULL);
>  }
>  
> -#define KVM_GUESTDBG_VALID_MASK (KVM_GUESTDBG_ENABLE)
> +#define KVM_GUESTDBG_VALID_MASK (KVM_GUESTDBG_ENABLE | KVM_GUESTDBG_USE_SW_BP)
>  
>  /**
>   * kvm_arch_vcpu_ioctl_set_guest_debug - set up guest debugging
> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> index b1f8731..5bee676 100644
> --- a/arch/arm64/kvm/debug.c
> +++ b/arch/arm64/kvm/debug.c
> @@ -75,6 +75,12 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
>  	else
>  		vcpu->arch.mdcr_el2 &= ~MDCR_EL2_TDA;
>  
> +	/* Trap breakpoints? */
> +	if (vcpu->guest_debug & KVM_GUESTDBG_USE_SW_BP)
> +		vcpu->arch.mdcr_el2 |= MDCR_EL2_TDE;
> +	else
> +		vcpu->arch.mdcr_el2 &= ~MDCR_EL2_TDE;
> +

I also don't think you need the else-clause here.

>  }
>  
>  void kvm_arm_clear_debug(struct kvm_vcpu *vcpu)
> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> index 524fa25..27f38a9 100644
> --- a/arch/arm64/kvm/handle_exit.c
> +++ b/arch/arm64/kvm/handle_exit.c
> @@ -82,6 +82,40 @@ static int kvm_handle_wfx(struct kvm_vcpu *vcpu, struct kvm_run *run)
>  	return 1;
>  }
>  
> +/**
> + * kvm_handle_guest_debug - handle a debug exception instruction
> + *
> + * @vcpu:	the vcpu pointer
> + * @run:	access to the kvm_run structure for results
> + *
> + * We route all debug exceptions through the same handler. If both the
> + * guest and host are using the same debug facilities it will be up to
> + * userspace to re-inject the correct exception for guest delivery.
> + *
> + * @return: 0 (while setting run->exit_reason), -1 for error
> + */
> +static int kvm_handle_guest_debug(struct kvm_vcpu *vcpu, struct kvm_run *run)
> +{
> +	u32 hsr = kvm_vcpu_get_hsr(vcpu);
> +	int ret = 0;
> +
> +	run->exit_reason = KVM_EXIT_DEBUG;
> +	run->debug.arch.hsr = hsr;
> +
> +	switch (hsr >> ESR_ELx_EC_SHIFT) {
> +	case ESR_ELx_EC_BKPT32:
> +	case ESR_ELx_EC_BRK64:
> +		break;
> +	default:
> +		kvm_err("%s: un-handled case hsr: %#08x\n",
> +			__func__, (unsigned int) hsr);
> +		ret = -1;
> +		break;
> +	}
> +
> +	return ret;
> +}
> +
>  static exit_handle_fn arm_exit_handlers[] = {
>  	[ESR_ELx_EC_WFx]	= kvm_handle_wfx,
>  	[ESR_ELx_EC_CP15_32]	= kvm_handle_cp15_32,
> @@ -96,6 +130,8 @@ static exit_handle_fn arm_exit_handlers[] = {
>  	[ESR_ELx_EC_SYS64]	= kvm_handle_sys_reg,
>  	[ESR_ELx_EC_IABT_LOW]	= kvm_handle_guest_abort,
>  	[ESR_ELx_EC_DABT_LOW]	= kvm_handle_guest_abort,
> +	[ESR_ELx_EC_BKPT32]	= kvm_handle_guest_debug,
> +	[ESR_ELx_EC_BRK64]	= kvm_handle_guest_debug,
>  };
>  
>  static exit_handle_fn kvm_get_exit_handler(struct kvm_vcpu *vcpu)
> -- 
> 2.3.5
> 

Besides the nit:

Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>