KVM: arm64: Clear PSTATE.SS when the Software Step state was Active-pending

Bugzilla: https://bugzilla.redhat.com/2128973

commit 370531d
Author: Reiji Watanabe <reijiw@google.com>
Date:   Fri Sep 16 18:05:58 2022 -0700

    KVM: arm64: Clear PSTATE.SS when the Software Step state was Active-pending

    While userspace enables single-step, if the Software Step state at the
    last guest exit was "Active-pending", clear PSTATE.SS on guest entry
    to restore the state.

    Currently, KVM sets PSTATE.SS to 1 on every guest entry while userspace
    enables single-step for the vCPU (with KVM_GUESTDBG_SINGLESTEP).
    It means KVM always makes the vCPU's Software Step state
    "Active-not-pending" on the guest entry, which lets the VCPU perform
    single-step (then Software Step exception is taken). This could cause
    extra single-step (without returning to userspace) if the Software Step
    state at the last guest exit was "Active-pending" (i.e. the last
    exit was triggered by an asynchronous exception after the single-step
    is performed, but before the Software Step exception is taken.
    See "Figure D2-3 Software step state machine" and "D2.12.7 Behavior
    in the active-pending state" in ARM DDI 0487I.a for more info about
    this behavior).

    Fix this by clearing PSTATE.SS on guest entry if the Software Step state
    at the last exit was "Active-pending" so that KVM restore the state (and
    the exception is taken before further single-step is performed).

    Fixes: 337b99b ("KVM: arm64: guest debug, add support for single-step")
    Signed-off-by: Reiji Watanabe <reijiw@google.com>
    Signed-off-by: Marc Zyngier <maz@kernel.org>
    Link: https://lore.kernel.org/r/20220917010600.532642-3-reijiw@google.com

Signed-off-by: Cornelia Huck <cohuck@redhat.com>

Author: cohuck

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) +	\

arch/arm64/kvm/debug.c

@@ -188,7 +188,18 @@ 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;
+			else
+				*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);
@@ -262,6 +273,15 @@ void kvm_arm_clear_debug(struct kvm_vcpu *vcpu)
 	 * Restore the guest's debug registers if we were using them.
 	 */
 	if (vcpu->guest_debug || kvm_vcpu_os_lock_enabled(vcpu)) {
+		if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
+			if (!(*vcpu_cpsr(vcpu) & DBG_SPSR_SS))
+				/*
+				 * Mark the vcpu as ACTIVE_PENDING
+				 * until Software Step exception is taken.
+				 */
+				vcpu_set_flag(vcpu, DBG_SS_ACTIVE_PENDING);
+		}
+
 		restore_guest_debug_regs(vcpu);
 
 		/*

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:

arch/arm64/kvm/handle_exit.c

@@ -152,8 +152,14 @@ static int kvm_handle_guest_debug(struct kvm_vcpu *vcpu)
 	run->debug.arch.hsr_high = upper_32_bits(esr);
 	run->flags = KVM_DEBUG_ARCH_HSR_HIGH_VALID;
 
-	if (ESR_ELx_EC(esr) == ESR_ELx_EC_WATCHPT_LOW)
+	switch (ESR_ELx_EC(esr)) {
+	case ESR_ELx_EC_WATCHPT_LOW:
 		run->debug.arch.far = vcpu->arch.fault.far_el2;
+		break;
+	case ESR_ELx_EC_SOFTSTP_LOW:
+		vcpu_clear_flag(vcpu, DBG_SS_ACTIVE_PENDING);
+		break;
+	}
 
 	return 0;
 }