[PATCH] xen: fix multicall debug feature

[Juergen Gross <jgross@xxxxxxxx>]

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: 368990a7fe30 ("xen: fix multicall debug data referencing")
Reported-by: Borislav Petkov <bp@xxxxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 arch/x86/xen/multicalls.c | 26 ++++++++++++++------------
 arch/x86/xen/smp_pv.c     |  1 -
 arch/x86/xen/xen-ops.h    |  3 ---
 3 files changed, 14 insertions(+), 16 deletions(-)

diff --git a/arch/x86/xen/multicalls.c b/arch/x86/xen/multicalls.c
index 10c660fae8b3..7237d56a9d3f 100644
--- a/arch/x86/xen/multicalls.c
+++ b/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;
diff --git a/arch/x86/xen/smp_pv.c b/arch/x86/xen/smp_pv.c
index 688ff59318ae..9bb8ff8bff30 100644
--- a/arch/x86/xen/smp_pv.c
+++ b/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
diff --git a/arch/x86/xen/xen-ops.h b/arch/x86/xen/xen-ops.h
index 63c13a2ccf55..25e318ef27d6 100644
--- a/arch/x86/xen/xen-ops.h
+++ b/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);
-- 
2.43.0