Re: [PATCH v6 10/12] KVM: arm64: guest debug, HW assisted debug support

[Christoffer Dall <christoffer.dall-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>]

On Fri, Jun 19, 2015 at 01:23:48PM +0100, Alex Bennée wrote:
> This adds support for userspace to control the HW debug registers for
> guest debug. In the debug ioctl we copy the IMPDEF defined number of

s/defined//

> registers into a new register set called host_debug_state. There is now
> a new vcpu parameter called debug_ptr which selects which register set
> is to copied into the real registers when world switch occurs.

But this patch doesn't seem to add the debug_ptr field?

s/to//

> 
> I've moved some helper functions into the hw_breakpoint.h header for
> re-use.
> 
> As with single step we need to tweak the guest registers to enable the
> exceptions so we need to save and restore those bits.
> 
> Two new capabilities have been added to the KVM_EXTENSION ioctl to allow
> userspace to query the number of hardware break and watch points
> available on the host hardware.
> 
> Signed-off-by: Alex Bennée <alex.bennee-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
> 
> ---
> v2
>    - switched to C setup
>    - replace host debug registers directly into context
>    - minor tweak to api docs
>    - setup right register for debug
>    - add FAR_EL2 to debug exit structure
>    - add support for trapping debug register access
> v3
>    - remove stray trace statement
>    - fix spacing around operators (various)
>    - clean-up usage of trap_debug
>    - introduce debug_ptr, replace excessive memcpy stuff
>    - don't use memcpy in ioctl, just assign
>    - update cap ioctl documentation
>    - reword a number comments
>    - rename host_debug_state->external_debug_state
> v4
>    - use the new u32/u64 split debug_ptr approach
>    - fix some wording/comments
> v5
>    - don't set MDSCR_EL1.KDE (not needed)
> v6
>    - update wording given change in commentary
>    - KVM_GUESTDBG_USE_HW_BP->KVM_GUESTDBG_USE_HW
> ---
>  Documentation/virtual/kvm/api.txt      |  7 ++++++-
>  arch/arm/kvm/arm.c                     |  7 +++++++
>  arch/arm64/include/asm/hw_breakpoint.h | 12 +++++++++++
>  arch/arm64/include/asm/kvm_host.h      |  6 +++++-
>  arch/arm64/kernel/hw_breakpoint.c      | 12 -----------
>  arch/arm64/kvm/debug.c                 | 37 +++++++++++++++++++++++++++++-----
>  arch/arm64/kvm/handle_exit.c           |  6 ++++++
>  arch/arm64/kvm/reset.c                 | 12 +++++++++++
>  include/uapi/linux/kvm.h               |  2 ++
>  9 files changed, 82 insertions(+), 19 deletions(-)
> 
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index 33c8143..ada57df 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -2668,7 +2668,7 @@ 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, arm64]
> -  - KVM_GUESTDBG_USE_HW_BP:     using hardware breakpoints [x86, s390]
> +  - KVM_GUESTDBG_USE_HW_BP:     using hardware breakpoints [x86, s390, arm64]
>    - KVM_GUESTDBG_INJECT_DB:     inject DB type exception [x86]
>    - KVM_GUESTDBG_INJECT_BP:     inject BP type exception [x86]
>    - KVM_GUESTDBG_EXIT_PENDING:  trigger an immediate guest exit [s390]
> @@ -2683,6 +2683,11 @@ updated to the correct (supplied) values.
>  The second part of the structure is architecture specific and
>  typically contains a set of debug registers.
>  
> +For arm64 the number of debug registers is implementation defined and
> +can be determined by querying the KVM_CAP_GUEST_DEBUG_HW_BPS and
> +KVM_CAP_GUEST_DEBUG_HW_WPS capabilities which return a positive number
> +indicating the number of supported registers.
> +
>  When debug events exit the main run loop with the reason
>  KVM_EXIT_DEBUG with the kvm_debug_exit_arch part of the kvm_run
>  structure containing architecture specific debug information.
> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> index 0d17c7b..60c4045 100644
> --- a/arch/arm/kvm/arm.c
> +++ b/arch/arm/kvm/arm.c
> @@ -307,6 +307,7 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>  
>  #define KVM_GUESTDBG_VALID_MASK (KVM_GUESTDBG_ENABLE |    \
>  			    KVM_GUESTDBG_USE_SW_BP | \
> +			    KVM_GUESTDBG_USE_HW | \
>  			    KVM_GUESTDBG_SINGLESTEP)
>  
>  /**
> @@ -327,6 +328,12 @@ int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
>  
>  	if (dbg->control & KVM_GUESTDBG_ENABLE) {
>  		vcpu->guest_debug = dbg->control;
> +
> +		/* Hardware assisted Break and Watch points */
> +		if (vcpu->guest_debug & KVM_GUESTDBG_USE_HW) {
> +			vcpu->arch.external_debug_state = dbg->arch;
> +		}
> +
>  	} else {
>  		/* If not enabled clear all flags */
>  		vcpu->guest_debug = 0;
> diff --git a/arch/arm64/include/asm/hw_breakpoint.h b/arch/arm64/include/asm/hw_breakpoint.h
> index 52b484b..c450552 100644
> --- a/arch/arm64/include/asm/hw_breakpoint.h
> +++ b/arch/arm64/include/asm/hw_breakpoint.h
> @@ -130,6 +130,18 @@ static inline void ptrace_hw_copy_thread(struct task_struct *task)
>  }
>  #endif
>  
> +/* Determine number of BRP registers available. */
> +static inline int get_num_brps(void)
> +{
> +	return ((read_cpuid(ID_AA64DFR0_EL1) >> 12) & 0xf) + 1;
> +}
> +
> +/* Determine number of WRP registers available. */
> +static inline int get_num_wrps(void)
> +{
> +	return ((read_cpuid(ID_AA64DFR0_EL1) >> 20) & 0xf) + 1;
> +}
> +
>  extern struct pmu perf_ops_bp;
>  
>  #endif	/* __KERNEL__ */
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 9697daf..0a3ee7b 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -116,13 +116,17 @@ struct kvm_vcpu_arch {
>  	 * debugging the guest from the host and to maintain separate host and
>  	 * guest state during world switches. vcpu_debug_state are the debug
>  	 * registers of the vcpu as the guest sees them.  host_debug_state are
> -	 * the host registers which are saved and restored during world switches.
> +	 * the host registers which are saved and restored during
> +	 * world switches. external_debug_state contains the debug
> +	 * values we want to debugging the guest. This is set via the

nit: s/debugging/debug/

> +	 * KVM_SET_GUEST_DEBUG ioctl.
>  	 *
>  	 * debug_ptr points to the set of debug registers that should be loaded
>  	 * onto the hardware when running the guest.
>  	 */
>  	struct kvm_guest_debug_arch *debug_ptr;
>  	struct kvm_guest_debug_arch vcpu_debug_state;
> +	struct kvm_guest_debug_arch external_debug_state;
>  
>  	/* Pointer to host CPU context */
>  	kvm_cpu_context_t *host_cpu_context;
> diff --git a/arch/arm64/kernel/hw_breakpoint.c b/arch/arm64/kernel/hw_breakpoint.c
> index e7d934d..3a41bbf 100644
> --- a/arch/arm64/kernel/hw_breakpoint.c
> +++ b/arch/arm64/kernel/hw_breakpoint.c
> @@ -49,18 +49,6 @@ static DEFINE_PER_CPU(int, stepping_kernel_bp);
>  static int core_num_brps;
>  static int core_num_wrps;
>  
> -/* Determine number of BRP registers available. */
> -static int get_num_brps(void)
> -{
> -	return ((read_cpuid(ID_AA64DFR0_EL1) >> 12) & 0xf) + 1;
> -}
> -
> -/* Determine number of WRP registers available. */
> -static int get_num_wrps(void)
> -{
> -	return ((read_cpuid(ID_AA64DFR0_EL1) >> 20) & 0xf) + 1;
> -}
> -
>  int hw_breakpoint_slots(int type)
>  {
>  	/*
> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> index d439eb8..b287bbc 100644
> --- a/arch/arm64/kvm/debug.c
> +++ b/arch/arm64/kvm/debug.c
> @@ -96,10 +96,6 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
>  				MDCR_EL2_TDRA |
>  				MDCR_EL2_TDOSA);
>  
> -	/* Trap on access to debug registers? */
> -	if (trap_debug)
> -		vcpu->arch.mdcr_el2 |= MDCR_EL2_TDA;
> -
>  	/* Is Guest debugging in effect? */
>  	if (vcpu->guest_debug) {
>  		/* Route all software debug exceptions to EL2 */
> @@ -134,11 +130,42 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
>  		} else {
>  			vcpu_sys_reg(vcpu, MDSCR_EL1) &= ~DBG_MDSCR_SS;
>  		}
> +
> +		/*
> +		 * HW Breakpoints and watchpoints
> +		 *
> +		 * We simply switch the debug_ptr to point to our new
> +		 * external_debug_state which has been populated by the
> +		 * debug ioctl. The existing KVM_ARM64_DEBUG_DIRTY
> +		 * mechanism ensures the registers are updated on the
> +		 * world switch.
> +		 */
> +		if (vcpu->guest_debug & KVM_GUESTDBG_USE_HW) {
> +			/* Enable breakpoints/watchpoints */
> +			vcpu_sys_reg(vcpu, MDSCR_EL1) |= DBG_MDSCR_MDE;
> +
> +			vcpu->arch.debug_ptr = &vcpu->arch.external_debug_state;
> +			vcpu->arch.debug_flags |= KVM_ARM64_DEBUG_DIRTY;
> +			trap_debug = true;
> +		}
>  	}
> +
> +	/* Trap debug register access */
> +	if (trap_debug)
> +		vcpu->arch.mdcr_el2 |= MDCR_EL2_TDA;
>  }
>  
>  void kvm_arm_clear_debug(struct kvm_vcpu *vcpu)
>  {
> -	if (vcpu->guest_debug)
> +	if (vcpu->guest_debug) {
>  		restore_guest_debug_regs(vcpu);
> +
> +		/*
> +		 * If we were using HW debug we need to restore the
> +		 * debug_ptr to the guest debug state.
> +		 */
> +		if (vcpu->guest_debug & KVM_GUESTDBG_USE_HW)
> +			vcpu->arch.debug_ptr = &vcpu->arch.vcpu_debug_state;

I still think this would be more cleanly done in the setup_debug
function, but ok:

Reviewed-by: Christoffer Dall <christoffer.dall-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>