Re: [PATCH v2 07/10] KVM: arm64: guest debug, add support for single-step

Andrew Jones
2015-04-09T13:24:43Z

On Tue, Mar 31, 2015 at 04:08:05PM +0100, Alex Bennée wrote:
> This adds support for single-stepping the guest. As userspace can and
> will manipulate guest registers before restarting any tweaking of the
> registers has to occur just before control is passed back to the guest.
> Furthermore while guest debugging is in effect we need to squash the
> ability of the guest to single-step itself as we have no easy way of
> re-entering the guest after the exception has been delivered to the
> hypervisor.
>
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>
> ---
> v2
> - Move pstate/mdscr manipulation into C
> - don't export guest_debug to assembly
> - add accessor for saved_debug regs
> - tweak save/restore of mdscr_el1
>
> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> index d3bc8dc..c1ed8cb 100644
> --- a/arch/arm/kvm/arm.c
> +++ b/arch/arm/kvm/arm.c
> @@ -304,7 +304,21 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> kvm_arm_set_running_vcpu(NULL);
> }
>
> -#define KVM_GUESTDBG_VALID (KVM_GUESTDBG_ENABLE|KVM_GUESTDBG_USE_SW_BP)
> +#define KVM_GUESTDBG_VALID (KVM_GUESTDBG_ENABLE | \
> + KVM_GUESTDBG_USE_SW_BP | \
> + KVM_GUESTDBG_SINGLESTEP)
> +
> +/**
> + * kvm_arch_vcpu_ioctl_set_guest_debug - Setup guest debugging
> + * @kvm: pointer to the KVM struct
> + * @kvm_guest_debug: the ioctl data buffer
> + *
> + * This sets up the VM for guest debugging. Care has to be taken when
> + * manipulating guest registers as these will be set/cleared by the
> + * hyper-visor controller, typically before each kvm_run event. As a
> + * result modification of the guest registers needs to take place
> + * after they have been restored in the hyp.S trampoline code.
> + */
>
> int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
> struct kvm_guest_debug *dbg)
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 0631840..6a33647 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -121,6 +121,13 @@ struct kvm_vcpu_arch {
> * here.
> */
>
> + /* Registers pre any guest debug manipulations */
> + struct {
> + u32 pstate_ss_bit;
> + u32 mdscr_el1_bits;
> +
> + } debug_saved_regs;

Hmm, you have a struct called "regs", but then each member is
suffixed with _bit(s). This looks awkward.

> +
> /* Don't run the guest */
> bool pause;
>
> @@ -143,6 +150,7 @@ struct kvm_vcpu_arch {
>
> #define vcpu_gp_regs(v) (&(v)->arch.ctxt.gp_regs)
> #define vcpu_sys_reg(v,r) ((v)->arch.ctxt.sys_regs[(r)])
> +#define vcpu_debug_saved_reg(v, r) ((v)->arch.debug_saved_regs.r)
> /*
> * CP14 and CP15 live in the same array, as they are backed by the
> * same system registers.
> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> index cff0475..b32362c 100644
> --- a/arch/arm64/kvm/debug.c
> +++ b/arch/arm64/kvm/debug.c
> @@ -19,8 +19,16 @@
>
> #include <linux/kvm_host.h>
>
> +#include <asm/debug-monitors.h>
> +#include <asm/kvm_asm.h>
> #include <asm/kvm_arm.h>
> #include <asm/kvm_host.h>
> +#include <asm/kvm_emulate.h>
> +
> +/* These are the bits of MDSCR_EL1 we may mess with */
> +#define MDSCR_EL1_DEBUG_BITS (DBG_MDSCR_SS | \
> + DBG_MDSCR_KDE | \
> + DBG_MDSCR_MDE)

_MASK instead of _BITS ?

>
> /**
> * kvm_arch_setup_debug - set-up debug related stuff
> @@ -51,15 +59,46 @@ void kvm_arch_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)
> + /* Is Guest debugging in effect? */
> + if (vcpu->guest_debug) {
> vcpu->arch.mdcr_el2 |= MDCR_EL2_TDE;
> - else
> - vcpu->arch.mdcr_el2 &= ~MDCR_EL2_TDE;
>
> + /* Save pstate/mdscr */
> + vcpu_debug_saved_reg(vcpu, pstate_ss_bit) =
> + *vcpu_cpsr(vcpu) & DBG_SPSR_SS;
> + vcpu_debug_saved_reg(vcpu, mdscr_el1_bits) =
> + vcpu_sys_reg(vcpu, MDSCR_EL1) & MDSCR_EL1_DEBUG_BITS;

I think it would be clearer if we embed the masks into helper
functions, and, assuming we drop the _bits concept too, then

#define SPSR_DEBUG_MASK DBG_SPSR_SS

vcpu_debug_save_regs(vcpu)
{
vcpu->arch.debug_saved_regs.pstate = *vcpu_cpsr(vcpu);
vcpu->arch.debug_saved_regs.mdscr_el1 = vcpu_sys_reg(vcpu, MDSCR_EL1);
}

vcpu_debug_restore_regs(vcpu)
{
*vcpu_cpsr(vcpu) |=
(vcpu->arch.debug_saved_regs.pstate & SPSR_DEBUG_MASK);
vcpu_sys_reg(vcpu, MDSCR_EL1) |=
(vcpu->arch.debug_saved_regs.mdscr_el1 & MDSCR_EL1_DEBUG_MASK)
}

> + /*
> + * Single Step (ARM ARM D2.12.3 The software step state
> + * machine)
> + *
> + * If we are doing Single Step we need to manipulate
> + * MDSCR_EL1.SS and PSTATE.SS. If not we need to
> + * suppress the guest from messing with it.
> + */
> + if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
> + *vcpu_cpsr(vcpu) |= DBG_SPSR_SS;
> + vcpu_sys_reg(vcpu, MDSCR_EL1) |= DBG_MDSCR_SS;
> + } else {
> + *vcpu_cpsr(vcpu) &= ~DBG_SPSR_SS;
> + vcpu_sys_reg(vcpu, MDSCR_EL1) &= ~DBG_MDSCR_SS;
> + }
> +
> + } else {
> + /* Debug operations can go straight to the guest */
> + vcpu->arch.mdcr_el2 &= ~MDCR_EL2_TDE;
> + }
> }
>
> void kvm_arch_clear_debug(struct kvm_vcpu *vcpu)
> {
> - /* Nothing to do yet */

This would now just be

if (vcpu->guest_debug)
vcpu_debug_restore_regs(vcpu);

> + if (vcpu->guest_debug) {
> + /* Restore pstate/mdscr bits we may have messed with */
> + *vcpu_cpsr(vcpu) &= ~DBG_SPSR_SS;
> + *vcpu_cpsr(vcpu) |= vcpu_debug_saved_reg(vcpu, pstate_ss_bit);
> +
> + vcpu_sys_reg(vcpu, MDSCR_EL1) &= ~MDSCR_EL1_DEBUG_BITS;
> + vcpu_sys_reg(vcpu, MDSCR_EL1) |=
> + vcpu_debug_saved_reg(vcpu, mdscr_el1_bits);
> + }
> }
> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> index ed1bbb4..16accae 100644
> --- a/arch/arm64/kvm/handle_exit.c
> +++ b/arch/arm64/kvm/handle_exit.c
> @@ -101,6 +101,7 @@ 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_SOFTSTP_LOW:
> case ESR_ELx_EC_BKPT32:
> case ESR_ELx_EC_BRK64:
> run->debug.arch.pc = *vcpu_pc(vcpu);
> @@ -127,6 +128,7 @@ 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_SOFTSTP_LOW]= kvm_handle_guest_debug,
> [ESR_ELx_EC_BKPT32] = kvm_handle_guest_debug,
> [ESR_ELx_EC_BRK64] = kvm_handle_guest_debug,
> };
> --
> 2.3.4
>