arm64: KVM: Consistently advance singlestep when emulating instructions

When we emulate a guest instruction, we don't advance the hardware
singlestep state machine, and thus the guest will receive a software
step exception after a next instruction which is not emulated by the
host.

We bodge around this in an ad-hoc fashion. Sometimes we explicitly check
whether userspace requested a single step, and fake a debug exception
from within the kernel. Other times, we advance the HW singlestep state
rely on the HW to generate the exception for us. Thus, the observed step
behaviour differs for host and guest.

Let's make this simpler and consistent by always advancing the HW
singlestep state machine when we skip an instruction. Thus we can rely
on the hardware to generate the singlestep exception for us, and never
need to explicitly check for an active-pending step, nor do we need to
fake a debug exception from the guest.

Cc: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

1 files changed, 4 insertions, 39 deletions

diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
index 7cc175c88a37..4282f05771c1 100644
--- a/arch/arm64/kvm/hyp/switch.c
+++ b/arch/arm64/kvm/hyp/switch.c
@@ -305,33 +305,6 @@ static bool __hyp_text __populate_fault_info(struct kvm_vcpu *vcpu)
 	return true;
 }
 
-/* Skip an instruction which has been emulated. Returns true if
- * execution can continue or false if we need to exit hyp mode because
- * single-step was in effect.
- */
-static bool __hyp_text __skip_instr(struct kvm_vcpu *vcpu)
-{
-	*vcpu_pc(vcpu) = read_sysreg_el2(elr);
-
-	if (vcpu_mode_is_32bit(vcpu)) {
-		vcpu->arch.ctxt.gp_regs.regs.pstate = read_sysreg_el2(spsr);
-		kvm_skip_instr32(vcpu, kvm_vcpu_trap_il_is32bit(vcpu));
-		write_sysreg_el2(vcpu->arch.ctxt.gp_regs.regs.pstate, spsr);
-	} else {
-		*vcpu_pc(vcpu) += 4;
-	}
-
-	write_sysreg_el2(*vcpu_pc(vcpu), elr);
-
-	if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
-		vcpu->arch.fault.esr_el2 =
-			(ESR_ELx_EC_SOFTSTP_LOW << ESR_ELx_EC_SHIFT) | 0x22;
-		return false;
-	} else {
-		return true;
-	}
-}
-
 static bool __hyp_text __hyp_switch_fpsimd(struct kvm_vcpu *vcpu)
 {
 	struct user_fpsimd_state *host_fpsimd = vcpu->arch.host_fpsimd_state;
@@ -420,20 +393,12 @@ static bool __hyp_text fixup_guest_exit(struct kvm_vcpu *vcpu, u64 *exit_code)
 		if (valid) {
 			int ret = __vgic_v2_perform_cpuif_access(vcpu);
 
-			if (ret ==  1 && __skip_instr(vcpu))
+			if (ret == 1)
 				return true;
 
-			if (ret == -1) {
-				/* Promote an illegal access to an
-				 * SError. If we would be returning
-				 * due to single-step clear the SS
-				 * bit so handle_exit knows what to
-				 * do after dealing with the error.
-				 */
-				if (!__skip_instr(vcpu))
-					*vcpu_cpsr(vcpu) &= ~DBG_SPSR_SS;
+			/* Promote an illegal access to an SError.*/
+			if (ret == -1)
 				*exit_code = ARM_EXCEPTION_EL1_SERROR;
-			}
 
 			goto exit;
 		}
@@ -444,7 +409,7 @@ static bool __hyp_text fixup_guest_exit(struct kvm_vcpu *vcpu, u64 *exit_code)
 	     kvm_vcpu_trap_get_class(vcpu) == ESR_ELx_EC_CP15_32)) {
 		int ret = __vgic_v3_perform_cpuif_access(vcpu);
 
-		if (ret == 1 && __skip_instr(vcpu))
+		if (ret == 1)
 			return true;
 	}