Re: [PATCH v2 06/10] KVM: arm64: guest debug, add SW break point support

[David Hildenbrand <dahi@linux.vnet.ibm.com>]

> This adds support for SW breakpoints inserted by userspace.
> 
> We do this by trapping all BKPT exceptions in the
> hypervisor (MDCR_EL2_TDE). The kvm_debug_exit_arch carries the address
> of the exception. If user-space doesn't know of the breakpoint then we
> have a guest inserted breakpoint and the hypervisor needs to start again
> and deliver the exception to guest.
> 
> 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
> 
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index 06c5064..17d4f9c 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -2626,7 +2626,7 @@ 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 7ea8b0e..d3bc8dc 100644
> --- a/arch/arm/kvm/arm.c
> +++ b/arch/arm/kvm/arm.c
> @@ -304,7 +304,7 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>  	kvm_arm_set_running_vcpu(NULL);
>  }
> 
> -#define KVM_GUESTDBG_VALID (KVM_GUESTDBG_ENABLE)
> +#define KVM_GUESTDBG_VALID (KVM_GUESTDBG_ENABLE|KVM_GUESTDBG_USE_SW_BP)
> 
>  int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
>  					struct kvm_guest_debug *dbg)
> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> index 8a29d0b..cff0475 100644
> --- a/arch/arm64/kvm/debug.c
> +++ b/arch/arm64/kvm/debug.c
> @@ -45,11 +45,18 @@ void kvm_arch_setup_debug(struct kvm_vcpu *vcpu)
>  	vcpu->arch.mdcr_el2 |= (MDCR_EL2_TPM | MDCR_EL2_TPMCR);
>  	vcpu->arch.mdcr_el2 |= (MDCR_EL2_TDRA | MDCR_EL2_TDOSA);
> 
> +	/* Trap debug register access? */

This should probably go to the earlier patch.

>  	if (!vcpu->arch.debug_flags & KVM_ARM64_DEBUG_DIRTY)
>  		vcpu->arch.mdcr_el2 |= MDCR_EL2_TDA;
>  	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;

Again, a candidate for clear_debug?

> +
>  }
> 
>  void kvm_arch_clear_debug(struct kvm_vcpu *vcpu)
> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> index 524fa25..ed1bbb4 100644
> --- a/arch/arm64/kvm/handle_exit.c
> +++ b/arch/arm64/kvm/handle_exit.c
> @@ -82,6 +82,37 @@ static int kvm_handle_wfx(struct kvm_vcpu *vcpu, struct kvm_run *run)
>  	return 1;
>  }
> 
> +/**
> + * kvm_handle_debug_exception - handle a debug exception instruction

"kvm_handle_guest_debug"

> + *
> + * @vcpu:	the vcpu pointer
> + * @run:	access to the kvm_run structure for results
> + *
> + * We route all debug exceptions through the same handler as we
> + * just need to report the PC and the HSR values to userspace.
> + * Userspace may decide to re-inject the exception and deliver it to
> + * the guest if it wasn't for the host to deal with.
> + */
> +static int kvm_handle_guest_debug(struct kvm_vcpu *vcpu, struct kvm_run *run)
> +{
> +	u32 hsr = kvm_vcpu_get_hsr(vcpu);
> +
> +	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:
> +		run->debug.arch.pc = *vcpu_pc(vcpu);
> +		break;
> +	default:
> +		kvm_err("%s: un-handled case hsr: %#08x\n",
> +			__func__, (unsigned int) hsr);

Don't you want to fail hard in this case? This might result in many messages.
returning 0 feels wrong.

> +		break;
> +	}
> +	return 0;
> +}
> +
>  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 +127,8 @@ 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)


David