KVM/arm fixes for 5.8, take #3

- Disable preemption on context-switching PMU EL0 state happening
   on system register trap
 - Don't clobber X0 when tearing down KVM via a soft reset (kexec)
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Merge tag 'kvmarm-fixes-5.8-3' of git://git.kernel.org/pub/scm/linux/kernel/git/kvmarm/kvmarm into kvm-master

KVM/arm fixes for 5.8, take #3

- Disable preemption on context-switching PMU EL0 state happening
  on system register trap
- Don't clobber X0 when tearing down KVM via a soft reset (kexec)

arch/arm64/kvm/hyp-init.S

@@ -136,11 +136,15 @@ SYM_CODE_START(__kvm_handle_stub_hvc)
 
 1:	cmp	x0, #HVC_RESET_VECTORS
 	b.ne	1f
-reset:
+
 	/*
-	 * Reset kvm back to the hyp stub. Do not clobber x0-x4 in
-	 * case we coming via HVC_SOFT_RESTART.
+	 * Set the HVC_RESET_VECTORS return code before entering the common
+	 * path so that we do not clobber x0-x2 in case we are coming via
+	 * HVC_SOFT_RESTART.
 	 */
+	mov	x0, xzr
+reset:
+	/* Reset kvm back to the hyp stub. */
 	mrs	x5, sctlr_el2
 	mov_q	x6, SCTLR_ELx_FLAGS
 	bic	x5, x5, x6		// Clear SCTL_M and etc
@@ -151,7 +155,6 @@ reset:
 	/* Install stub vectors */
 	adr_l	x5, __hyp_stub_vectors
 	msr	vbar_el2, x5
-	mov	x0, xzr
 	eret
 
 1:	/* Bad stub call */

arch/arm64/kvm/pmu.c

@@ -159,7 +159,10 @@ static void kvm_vcpu_pmu_disable_el0(unsigned long events)
 }
 
 /*
- * On VHE ensure that only guest events have EL0 counting enabled
+ * On VHE ensure that only guest events have EL0 counting enabled.
+ * This is called from both vcpu_{load,put} and the sysreg handling.
+ * Since the latter is preemptible, special care must be taken to
+ * disable preemption.
  */
 void kvm_vcpu_pmu_restore_guest(struct kvm_vcpu *vcpu)
 {
@@ -169,12 +172,14 @@ void kvm_vcpu_pmu_restore_guest(struct kvm_vcpu *vcpu)
 	if (!has_vhe())
 		return;
 
+	preempt_disable();
 	host = this_cpu_ptr(&kvm_host_data);
 	events_guest = host->pmu_events.events_guest;
 	events_host = host->pmu_events.events_host;
 
 	kvm_vcpu_pmu_enable_el0(events_guest);
 	kvm_vcpu_pmu_disable_el0(events_host);
+	preempt_enable();
 }
 
 /*