powerpc: Fix is_kvm_guest() / kvm_para_available()

Commit a21d1bec ("powerpc: Reintroduce is_kvm_guest() as a fast-path
check") added is_kvm_guest() and changed kvm_para_available() to use it.

is_kvm_guest() checks a static key, kvm_guest, and that static key is
set in check_kvm_guest().

The problem is check_kvm_guest() is only called on pseries, and even
then only in some configurations. That means is_kvm_guest() always
returns false on all non-pseries and some pseries depending on
configuration. That's a bug.

For PR KVM guests this is noticable because they no longer do live
patching of themselves, which can be detected by the omission of a
message in dmesg such as:

  KVM: Live patching for a fast VM worked

To fix it make check_kvm_guest() an initcall, to ensure it's always
called at boot. It needs to be core so that it runs before
kvm_guest_init() which is postcore. To be an initcall it needs to return
int, where 0 means success, so update that.

We still call it manually in pSeries_smp_probe(), because that runs
before init calls are run.

Fixes: a21d1bec ("powerpc: Reintroduce is_kvm_guest() as a fast-path check")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210623130514.2543232-1-mpe@ellerman.id.au

arch/powerpc/include/asm/kvm_guest.h

@@ -16,10 +16,10 @@ static inline bool is_kvm_guest(void)
 	return static_branch_unlikely(&kvm_guest);
 }
 
-bool check_kvm_guest(void);
+int check_kvm_guest(void);
 #else
 static inline bool is_kvm_guest(void) { return false; }
-static inline bool check_kvm_guest(void) { return false; }
+static inline int check_kvm_guest(void) { return 0; }
 #endif
 
 #endif /* _ASM_POWERPC_KVM_GUEST_H_ */

arch/powerpc/kernel/firmware.c

@@ -23,18 +23,20 @@ EXPORT_SYMBOL_GPL(powerpc_firmware_features);
 
 #if defined(CONFIG_PPC_PSERIES) || defined(CONFIG_KVM_GUEST)
 DEFINE_STATIC_KEY_FALSE(kvm_guest);
-bool check_kvm_guest(void)
+int __init check_kvm_guest(void)
 {
 	struct device_node *hyper_node;
 
 	hyper_node = of_find_node_by_path("/hypervisor");
 	if (!hyper_node)
-		return false;
+		return 0;
 
 	if (!of_device_is_compatible(hyper_node, "linux,kvm"))
-		return false;
+		return 0;
 
 	static_branch_enable(&kvm_guest);
-	return true;
+
+	return 0;
 }
+core_initcall(check_kvm_guest); // before kvm_guest_init()
 #endif

arch/powerpc/platforms/pseries/smp.c

@@ -211,7 +211,9 @@ static __init void pSeries_smp_probe(void)
 	if (!cpu_has_feature(CPU_FTR_SMT))
 		return;
 
-	if (check_kvm_guest()) {
+	check_kvm_guest();
+
+	if (is_kvm_guest()) {
 		/*
 		 * KVM emulates doorbells by disabling FSCR[MSGP] so msgsndp
 		 * faults to the hypervisor which then reads the instruction