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SubjectRe: [PATCH v3 09/12] KVM: arm64: guest debug, HW assisted debug support
On Thu, May 07, 2015 at 10:07:12AM +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
> 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.
>
> 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@linaro.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
>
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index 5ef937c..419f7a8 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 returns a +ve number

s/returns/return/
s/+ve/positive/

> +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 9b3ed6d..2920185 100644
> --- a/arch/arm/kvm/arm.c
> +++ b/arch/arm/kvm/arm.c
> @@ -279,6 +279,10 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
> /* Set up the timer */
> kvm_timer_vcpu_init(vcpu);
>
> + /* Set the debug registers to be the guests */
> + vcpu->arch.debug_ptr = (struct kvm_guest_debug_arch *)
> + &vcpu_sys_reg(vcpu, DBGBCR0_EL1);
> +

yikes, I don't like this cast, how bad is it to get rid of the debug
registers in the sys_regs array ?

Also, pretty sure this is part of the breakage for the 32-bit build...

> return 0;
> }
>
> @@ -304,6 +308,7 @@ 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_BP | \
> KVM_GUESTDBG_SINGLESTEP)
>
> /**
> @@ -324,6 +329,12 @@ 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_BP) {

is this only breakpoints or breakpoints and watch points?

> + 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;
> +}
> +

I will need an ack from Catalin/Will to merge this. It may be better to
move these functions in a separate patch.

> 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 b60fa7a..a44fb32 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -108,9 +108,18 @@ struct kvm_vcpu_arch {
> /* Exception Information */
> struct kvm_vcpu_fault_info fault;
>
> - /* Debug state */
> + /* Guest debug state */
> u64 debug_flags;
>
> + /*
> + * For debugging the guest we need to keep a set of debug
> + * registers which can override the guests own debug state

s/guests/guest's/

> + * while being used. These are set via the KVM_SET_GUEST_DEBUG
> + * ioctl.
> + */
> + struct kvm_guest_debug_arch *debug_ptr;
> + 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/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
> index 04957d7..98e82ef 100644
> --- a/arch/arm64/include/uapi/asm/kvm.h
> +++ b/arch/arm64/include/uapi/asm/kvm.h
> @@ -121,7 +121,7 @@ struct kvm_guest_debug_arch {
>
> struct kvm_debug_exit_arch {
> __u32 hsr;
> - __u64 far;
> + __u64 far; /* used for watchpoints */

seems strange to amend this now?

> };
>
> struct kvm_sync_regs {
> diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
> index ce7b7dd..671ab13 100644
> --- a/arch/arm64/kernel/asm-offsets.c
> +++ b/arch/arm64/kernel/asm-offsets.c
> @@ -116,6 +116,7 @@ int main(void)
> DEFINE(VCPU_FAR_EL2, offsetof(struct kvm_vcpu, arch.fault.far_el2));
> DEFINE(VCPU_HPFAR_EL2, offsetof(struct kvm_vcpu, arch.fault.hpfar_el2));
> DEFINE(VCPU_DEBUG_FLAGS, offsetof(struct kvm_vcpu, arch.debug_flags));
> + DEFINE(VCPU_DEBUG_PTR, offsetof(struct kvm_vcpu, arch.debug_ptr));
> DEFINE(DEBUG_BCR, offsetof(struct kvm_guest_debug_arch, dbg_bcr));
> DEFINE(DEBUG_BVR, offsetof(struct kvm_guest_debug_arch, dbg_bvr));
> DEFINE(DEBUG_WCR, offsetof(struct kvm_guest_debug_arch, dbg_wcr));
> 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 19346e8..1ab63dd 100644
> --- a/arch/arm64/kvm/debug.c
> +++ b/arch/arm64/kvm/debug.c
> @@ -99,12 +99,6 @@ 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;
> - else
> - vcpu->arch.mdcr_el2 &= ~MDCR_EL2_TDA;
> -
> /* Is Guest debugging in effect? */
> if (vcpu->guest_debug) {
> vcpu->arch.mdcr_el2 |= MDCR_EL2_TDE;
> @@ -128,14 +122,54 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
> vcpu_sys_reg(vcpu, MDSCR_EL1) &= ~DBG_MDSCR_SS;
> }
>
> + /*
> + * HW Break/Watch points
> + *
> + * 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_BP) {
> +
> + vcpu_sys_reg(vcpu, MDSCR_EL1) |=
> + (DBG_MDSCR_KDE | DBG_MDSCR_MDE);

Why do we need to set these two bits? Is it obvious or does it deserve
a comment?

> +
> + vcpu->arch.debug_ptr = &vcpu->arch.external_debug_state;
> + vcpu->arch.debug_flags |= KVM_ARM64_DEBUG_DIRTY;
> + trap_debug = true;
> + }
> +
> } else {
> /* Debug operations can go straight to the guest */
> vcpu->arch.mdcr_el2 &= ~MDCR_EL2_TDE;
> }
> +
> + /*
> + * If the guest debug register state is dirty (the guest is
> + * actively accessing them), then we context-switch the
> + * registers in EL2. Otherwise, we trap-and-emulate all guest
> + * accesses to them.
> + */

I think this comment now feels strange, because it was explaining why we
would set the trap_debug variable when the dirty flag was set, but the
code just sets TDA when trap_debug is set. So you should either move
this comment to the top of the function and have it above a separate
line that sets trap_debug based on KVM_ARM64_DEBUG_DIRTY (instead of
initializing at declaration), or you should explain which conditions set
trap_debug (guest is using the regs or we are debugging the guest), or
just get rid of the comment.

> + if (trap_debug)
> + vcpu->arch.mdcr_el2 |= MDCR_EL2_TDA;
> + else
> + vcpu->arch.mdcr_el2 &= ~MDCR_EL2_TDA;

still don't need the else.

> }
>
> 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_BP) {
> + vcpu->arch.debug_ptr = (struct kvm_guest_debug_arch *)
> + &vcpu_sys_reg(vcpu, DBGBCR0_EL1);
> + }

I would find it easier to follow the code if you only configure the
debug_ptr in kvm_arm_setup_debug() because it feels like you're setting
up state here which will not be used before in a very long time (after
handle_exit, exit to userspace etc.).

> + }
> }
> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> index e9de13e..68a0759 100644
> --- a/arch/arm64/kvm/handle_exit.c
> +++ b/arch/arm64/kvm/handle_exit.c
> @@ -103,7 +103,11 @@ static int kvm_handle_guest_debug(struct kvm_vcpu *vcpu, struct kvm_run *run)
> run->debug.arch.hsr = hsr;
>
> switch (hsr >> ESR_ELx_EC_SHIFT) {
> + case ESR_ELx_EC_WATCHPT_LOW:
> + run->debug.arch.far = vcpu->arch.fault.far_el2;
> + /* fall through */
> case ESR_ELx_EC_SOFTSTP_LOW:
> + case ESR_ELx_EC_BREAKPT_LOW:
> case ESR_ELx_EC_BKPT32:
> case ESR_ELx_EC_BRK64:
> break;
> @@ -132,6 +136,8 @@ static exit_handle_fn arm_exit_handlers[] = {
> [ESR_ELx_EC_IABT_LOW] = kvm_handle_guest_abort,
> [ESR_ELx_EC_DABT_LOW] = kvm_handle_guest_abort,
> [ESR_ELx_EC_SOFTSTP_LOW]= kvm_handle_guest_debug,
> + [ESR_ELx_EC_WATCHPT_LOW]= kvm_handle_guest_debug,
> + [ESR_ELx_EC_BREAKPT_LOW]= kvm_handle_guest_debug,
> [ESR_ELx_EC_BKPT32] = kvm_handle_guest_debug,
> [ESR_ELx_EC_BRK64] = kvm_handle_guest_debug,
> };
> diff --git a/arch/arm64/kvm/hyp.S b/arch/arm64/kvm/hyp.S
> index dd51fb1..921d248 100644
> --- a/arch/arm64/kvm/hyp.S
> +++ b/arch/arm64/kvm/hyp.S
> @@ -706,7 +706,8 @@ ENTRY(__kvm_vcpu_run)
> bl __restore_fpsimd
>
> skip_debug_state x3, 1f
> - add x3, x2, #CPU_SYSREG_OFFSET(DBGBCR0_EL1)
> + ldr x3, [x0, #VCPU_DEBUG_PTR]
> + kern_hyp_va x3
> bl __restore_debug
> 1:
> restore_guest_32bit_state
> @@ -727,7 +728,8 @@ __kvm_vcpu_return:
> bl __save_sysregs
>
> skip_debug_state x3, 1f
> - add x3, x2, #CPU_SYSREG_OFFSET(DBGBCR0_EL1)
> + ldr x3, [x0, #VCPU_DEBUG_PTR]
> + kern_hyp_va x3
> bl __save_debug
> 1:
> save_guest_32bit_state
> diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> index 0b43265..21d5a62 100644
> --- a/arch/arm64/kvm/reset.c
> +++ b/arch/arm64/kvm/reset.c
> @@ -56,6 +56,12 @@ static bool cpu_has_32bit_el1(void)
> return !!(pfr0 & 0x20);
> }
>
> +/**
> + * kvm_arch_dev_ioctl_check_extension
> + *
> + * We currently assume that the number of HW registers is uniform
> + * across all CPUs (see cpuinfo_sanity_check).
> + */
> int kvm_arch_dev_ioctl_check_extension(long ext)
> {
> int r;
> @@ -64,6 +70,12 @@ int kvm_arch_dev_ioctl_check_extension(long ext)
> case KVM_CAP_ARM_EL1_32BIT:
> r = cpu_has_32bit_el1();
> break;
> + case KVM_CAP_GUEST_DEBUG_HW_BPS:
> + r = get_num_brps();
> + break;
> + case KVM_CAP_GUEST_DEBUG_HW_WPS:
> + r = get_num_wrps();
> + break;
> default:
> r = 0;
> }
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 3b6252e..923c2aa 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -825,6 +825,8 @@ struct kvm_ppc_smmu_info {
> #define KVM_CAP_S390_INJECT_IRQ 113
> #define KVM_CAP_S390_IRQ_STATE 114
> #define KVM_CAP_PPC_HWRNG 115
> +#define KVM_CAP_GUEST_DEBUG_HW_BPS 116
> +#define KVM_CAP_GUEST_DEBUG_HW_WPS 117
>
> #ifdef KVM_CAP_IRQ_ROUTING
>
> --
> 2.3.5
>
Thanks,
-Christoffer


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