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

[Reiji Watanabe]

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 at 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