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

[Alex BennÃe]

Christoffer Dall <christoffer.dall@xxxxxxxxxx> writes:

> On Fri, Jun 19, 2015 at 01:23:48PM +0100, Alex BennÃe 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?

Oops, yes the comment belongs to the previous patch.

>
> 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 BennÃe <alex.bennee@xxxxxxxxxx>
>> 
>> ---
>> 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:

I don't follow, setup_debug is called before we enter KVM. It's pretty
light when no debugging is being done so this ensure we leave state how
we would like it when we stop debugging.

I can move it to an else leg in setup if you really want.

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

-- 
Alex BennÃe
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