xen: fix multicall debug feature

Initializing a percpu variable with the address of a struct tagged as
.initdata is breaking the build with CONFIG_SECTION_MISMATCH_WARN_ONLY
not set to "y".

Fix that by using an access function instead returning the .initdata
struct address if the percpu space of the struct hasn't been
allocated yet.

Fixes: 368990a ("xen: fix multicall debug data referencing")
Reported-by: Borislav Petkov <bp@alien8.de>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Acked-by: "Borislav Petkov (AMD)" <bp@alien8.de>
Tested-by: "Borislav Petkov (AMD)" <bp@alien8.de>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20250327190602.26015-1-jgross@suse.com>

arch/x86/xen/multicalls.c

@@ -54,14 +54,20 @@ struct mc_debug_data {
 
 static DEFINE_PER_CPU(struct mc_buffer, mc_buffer);
 static struct mc_debug_data mc_debug_data_early __initdata;
-static DEFINE_PER_CPU(struct mc_debug_data *, mc_debug_data) =
-	&mc_debug_data_early;
 static struct mc_debug_data __percpu *mc_debug_data_ptr;
 DEFINE_PER_CPU(unsigned long, xen_mc_irq_flags);
 
 static struct static_key mc_debug __ro_after_init;
 static bool mc_debug_enabled __initdata;
 
+static struct mc_debug_data * __ref get_mc_debug(void)
+{
+	if (!mc_debug_data_ptr)
+		return &mc_debug_data_early;
+
+	return this_cpu_ptr(mc_debug_data_ptr);
+}
+
 static int __init xen_parse_mc_debug(char *arg)
 {
 	mc_debug_enabled = true;
@@ -71,20 +77,16 @@ static int __init xen_parse_mc_debug(char *arg)
 }
 early_param("xen_mc_debug", xen_parse_mc_debug);
 
-void mc_percpu_init(unsigned int cpu)
-{
-	per_cpu(mc_debug_data, cpu) = per_cpu_ptr(mc_debug_data_ptr, cpu);
-}
-
 static int __init mc_debug_enable(void)
 {
 	unsigned long flags;
+	struct mc_debug_data __percpu *mcdb;
 
 	if (!mc_debug_enabled)
 		return 0;
 
-	mc_debug_data_ptr = alloc_percpu(struct mc_debug_data);
-	if (!mc_debug_data_ptr) {
+	mcdb = alloc_percpu(struct mc_debug_data);
+	if (!mcdb) {
 		pr_err("xen_mc_debug inactive\n");
 		static_key_slow_dec(&mc_debug);
 		return -ENOMEM;
@@ -93,7 +95,7 @@ static int __init mc_debug_enable(void)
 	/* Be careful when switching to percpu debug data. */
 	local_irq_save(flags);
 	xen_mc_flush();
-	mc_percpu_init(0);
+	mc_debug_data_ptr = mcdb;
 	local_irq_restore(flags);
 
 	pr_info("xen_mc_debug active\n");
@@ -155,7 +157,7 @@ void xen_mc_flush(void)
 	trace_xen_mc_flush(b->mcidx, b->argidx, b->cbidx);
 
 	if (static_key_false(&mc_debug)) {
-		mcdb = __this_cpu_read(mc_debug_data);
+		mcdb = get_mc_debug();
 		memcpy(mcdb->entries, b->entries,
 		       b->mcidx * sizeof(struct multicall_entry));
 	}
@@ -235,7 +237,7 @@ struct multicall_space __xen_mc_entry(size_t args)
 
 	ret.mc = &b->entries[b->mcidx];
 	if (static_key_false(&mc_debug)) {
-		struct mc_debug_data *mcdb = __this_cpu_read(mc_debug_data);
+		struct mc_debug_data *mcdb = get_mc_debug();
 
 		mcdb->caller[b->mcidx] = __builtin_return_address(0);
 		mcdb->argsz[b->mcidx] = args;

arch/x86/xen/smp_pv.c

@@ -305,7 +305,6 @@ static int xen_pv_kick_ap(unsigned int cpu, struct task_struct *idle)
 		return rc;
 
 	xen_pmu_init(cpu);
-	mc_percpu_init(cpu);
 
 	/*
 	 * Why is this a BUG? If the hypercall fails then everything can be

arch/x86/xen/xen-ops.h

@@ -261,9 +261,6 @@ void xen_mc_callback(void (*fn)(void *), void *data);
  */
 struct multicall_space xen_mc_extend_args(unsigned long op, size_t arg_size);
 
-/* Do percpu data initialization for multicalls. */
-void mc_percpu_init(unsigned int cpu);
-
 extern bool is_xen_pmu;
 
 irqreturn_t xen_pmu_irq_handler(int irq, void *dev_id);