[PATCH 1/3] KVM: arm64: Don't set PSTATE.SS when Software Step state is Active-pending

Reiji Watanabe
2022-09-09T04:47:07Z

Currently, PSTATE.SS is set on every guest entry if single-step is
enabled for the vCPU by userspace. However, it could cause extra
single-step execution without returning to userspace, which shouldn't
be performed, if the Software Step state at the last guest exit was
Active-pending (i.e. the last exit was not triggered by Software Step
exception, but by an asynchronous exception after the single-step
execution is performed).

Fix this by not setting PSTATE.SS on guest entry if the Software
Step state at the last exit was Active-pending.

Fixes: 337b99bf7edf ("KVM: arm64: guest debug, add support for single-step")
Signed-off-by: Reiji Watanabe <reijiw@google.com>
---
arch/arm64/include/asm/kvm_host.h | 3 +++
arch/arm64/kvm/debug.c | 19 ++++++++++++++++++-
arch/arm64/kvm/guest.c | 1 +
arch/arm64/kvm/handle_exit.c | 2 ++
4 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index e9c9388ccc02..4cf6eef02565 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -535,6 +535,9 @@ struct kvm_vcpu_arch {
#define IN_WFIT __vcpu_single_flag(sflags, BIT(3))
/* vcpu system registers loaded on physical CPU */
#define SYSREGS_ON_CPU __vcpu_single_flag(sflags, BIT(4))
+/* Software step state is Active-pending */
+#define DBG_SS_ACTIVE_PENDING __vcpu_single_flag(sflags, BIT(5))
+

/* Pointer to the vcpu's SVE FFR for sve_{save,load}_state() */
#define vcpu_sve_pffr(vcpu) (kern_hyp_va((vcpu)->arch.sve_state) + \
diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
index 0b28d7db7c76..125cfb94b4ad 100644
--- a/arch/arm64/kvm/debug.c
+++ b/arch/arm64/kvm/debug.c
@@ -188,7 +188,16 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
* debugging the system.
*/
if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
- *vcpu_cpsr(vcpu) |= DBG_SPSR_SS;
+ /*
+ * If the software step state at the last guest exit
+ * was Active-pending, we don't set DBG_SPSR_SS so
+ * that the state is maintained (to not run another
+ * single-step until the pending Software Step
+ * exception is taken).
+ */
+ if (!vcpu_get_flag(vcpu, DBG_SS_ACTIVE_PENDING))
+ *vcpu_cpsr(vcpu) |= DBG_SPSR_SS;
+
mdscr = vcpu_read_sys_reg(vcpu, MDSCR_EL1);
mdscr |= DBG_MDSCR_SS;
vcpu_write_sys_reg(vcpu, mdscr, MDSCR_EL1);
@@ -279,6 +288,14 @@ void kvm_arm_clear_debug(struct kvm_vcpu *vcpu)
&vcpu->arch.debug_ptr->dbg_wcr[0],
&vcpu->arch.debug_ptr->dbg_wvr[0]);
}
+
+ if ((vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) &&
+ !(*vcpu_cpsr(vcpu) & DBG_SPSR_SS))
+ /*
+ * Mark the vcpu as ACTIVE_PENDING
+ * until Software Step exception is confirmed.
+ */
+ vcpu_set_flag(vcpu, DBG_SS_ACTIVE_PENDING);
}
}

diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
index f802a3b3f8db..2ff13a3f8479 100644
--- a/arch/arm64/kvm/guest.c
+++ b/arch/arm64/kvm/guest.c
@@ -937,6 +937,7 @@ int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
} else {
/* If not enabled clear all flags */
vcpu->guest_debug = 0;
+ vcpu_clear_flag(vcpu, DBG_SS_ACTIVE_PENDING);
}

out:
diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
index bbe5b393d689..8e43b2668d67 100644
--- a/arch/arm64/kvm/handle_exit.c
+++ b/arch/arm64/kvm/handle_exit.c
@@ -154,6 +154,8 @@ static int kvm_handle_guest_debug(struct kvm_vcpu *vcpu)

if (ESR_ELx_EC(esr) == ESR_ELx_EC_WATCHPT_LOW)
run->debug.arch.far = vcpu->arch.fault.far_el2;
+ else if (ESR_ELx_EC(esr) == ESR_ELx_EC_SOFTSTP_LOW)
+ vcpu_clear_flag(vcpu, DBG_SS_ACTIVE_PENDING);

return 0;
}
--
2.37.2.789.g6183377224-goog