KVM: arm64: Preserve PSTATE.SS for the guest while single-step is enabled

Preserve the PSTATE.SS value for the guest while userspace enables
single-step (i.e. while KVM manipulates the PSTATE.SS) for the vCPU.

Currently, while userspace enables single-step for the vCPU
(with KVM_GUESTDBG_SINGLESTEP), KVM sets PSTATE.SS to 1 on every
guest entry, not saving its original value.
When userspace disables single-step, KVM doesn't restore the original
value for the subsequent guest entry (use the current value instead).
Exception return instructions copy PSTATE.SS from SPSR_ELx.SS
only in certain cases when single-step is enabled (and set it to 0
in other cases). So, the value matters only when the guest enables
single-step (and when the guest's Software step state isn't affected
by single-step enabled by userspace, practically), though.

Fix this by preserving the original PSTATE.SS value while userspace
enables single-step, and restoring the value once it is disabled.

This fix modifies the behavior of GET_ONE_REG/SET_ONE_REG for the
PSTATE.SS while single-step is enabled by userspace.
Presently, GET_ONE_REG/SET_ONE_REG gets/sets the current PSTATE.SS
value, which KVM will override on the next guest entry (i.e. the
value userspace gets/sets is not used for the next guest entry).
With this patch, GET_ONE_REG/SET_ONE_REG will get/set the guest's
preserved value, which KVM will preserve and try to restore after
single-step is disabled.

Fixes: 337b99bf ("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/20220917010600.532642-2-reijiw@google.com

arch/arm64/include/asm/kvm_host.h

@@ -393,6 +393,7 @@ struct kvm_vcpu_arch {
 	 */
 	struct {
 		u32	mdscr_el1;
+		bool	pstate_ss;
 	} guest_debug_preserved;
 
 	/* vcpu power state */

arch/arm64/kvm/debug.c

@@ -32,6 +32,10 @@ static DEFINE_PER_CPU(u64, mdcr_el2);
  *
  * Guest access to MDSCR_EL1 is trapped by the hypervisor and handled
  * after we have restored the preserved value to the main context.
+ *
+ * When single-step is enabled by userspace, we tweak PSTATE.SS on every
+ * guest entry. Preserve PSTATE.SS so we can restore the original value
+ * for the vcpu after the single-step is disabled.
  */
 static void save_guest_debug_regs(struct kvm_vcpu *vcpu)
 {
@@ -41,6 +45,9 @@ static void save_guest_debug_regs(struct kvm_vcpu *vcpu)
 
 	trace_kvm_arm_set_dreg32("Saved MDSCR_EL1",
 				vcpu->arch.guest_debug_preserved.mdscr_el1);
+
+	vcpu->arch.guest_debug_preserved.pstate_ss =
+					(*vcpu_cpsr(vcpu) & DBG_SPSR_SS);
 }
 
 static void restore_guest_debug_regs(struct kvm_vcpu *vcpu)
@@ -51,6 +58,11 @@ static void restore_guest_debug_regs(struct kvm_vcpu *vcpu)
 
 	trace_kvm_arm_set_dreg32("Restored MDSCR_EL1",
 				vcpu_read_sys_reg(vcpu, MDSCR_EL1));
+
+	if (vcpu->arch.guest_debug_preserved.pstate_ss)
+		*vcpu_cpsr(vcpu) |= DBG_SPSR_SS;
+	else
+		*vcpu_cpsr(vcpu) &= ~DBG_SPSR_SS;
 }
 
 /**