x86/boot: Skip realmode init code when running as Xen PV guest

When running as a Xen PV guest there is no need for setting up the
realmode trampoline, as realmode isn't supported in this environment.

Trying to setup the trampoline has been proven to be problematic in
some cases, especially when trying to debug early boot problems with
Xen requiring to keep the EFI boot-services memory mapped (some
firmware variants seem to claim basically all memory below 1Mb for boot
services).

Introduce new x86_platform_ops operations for that purpose, which can
be set to a NOP by the Xen PV specific kernel boot code.

  [ bp: s/call_init_real_mode/do_init_real_mode/ ]

Fixes: 084ee1c641 ("x86, realmode: Relocator for realmode code")
Suggested-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20221123114523.3467-1-jgross@suse.com

arch/x86/include/asm/realmode.h

@@ -91,6 +91,7 @@ static inline void set_real_mode_mem(phys_addr_t mem)
 
 void reserve_real_mode(void);
 void load_trampoline_pgtable(void);
+void init_real_mode(void);
 
 #endif /* __ASSEMBLY__ */
 

arch/x86/include/asm/x86_init.h

@@ -285,6 +285,8 @@ struct x86_hyper_runtime {
  * 				possible in x86_early_init_platform_quirks() by
  * 				only using the current x86_hardware_subarch
  * 				semantics.
+ * @realmode_reserve:		reserve memory for realmode trampoline
+ * @realmode_init:		initialize realmode trampoline
  * @hyper:			x86 hypervisor specific runtime callbacks
  */
 struct x86_platform_ops {
@@ -301,6 +303,8 @@ struct x86_platform_ops {
 	void (*apic_post_init)(void);
 	struct x86_legacy_features legacy;
 	void (*set_legacy_features)(void);
+	void (*realmode_reserve)(void);
+	void (*realmode_init)(void);
 	struct x86_hyper_runtime hyper;
 	struct x86_guest guest;
 };

arch/x86/kernel/setup.c

@@ -1175,7 +1175,7 @@ void __init setup_arch(char **cmdline_p)
 	 * Moreover, on machines with SandyBridge graphics or in setups that use
 	 * crashkernel the entire 1M is reserved anyway.
 	 */
-	reserve_real_mode();
+	x86_platform.realmode_reserve();
 
 	init_mem_mapping();
 

arch/x86/kernel/x86_init.c

@@ -25,6 +25,7 @@
 #include <asm/iommu.h>
 #include <asm/mach_traps.h>
 #include <asm/irqdomain.h>
+#include <asm/realmode.h>
 
 void x86_init_noop(void) { }
 void __init x86_init_uint_noop(unsigned int unused) { }
@@ -145,6 +146,8 @@ struct x86_platform_ops x86_platform __ro_after_init = {
 	.get_nmi_reason			= default_get_nmi_reason,
 	.save_sched_clock_state		= tsc_save_sched_clock_state,
 	.restore_sched_clock_state	= tsc_restore_sched_clock_state,
+	.realmode_reserve		= reserve_real_mode,
+	.realmode_init			= init_real_mode,
 	.hyper.pin_vcpu			= x86_op_int_noop,
 
 	.guest = {

arch/x86/realmode/init.c

@@ -200,14 +200,18 @@ static void __init set_real_mode_permissions(void)
 	set_memory_x((unsigned long) text_start, text_size >> PAGE_SHIFT);
 }
 
-static int __init init_real_mode(void)
+void __init init_real_mode(void)
 {
 	if (!real_mode_header)
 		panic("Real mode trampoline was not allocated");
 
 	setup_real_mode();
 	set_real_mode_permissions();
+}
 
+static int __init do_init_real_mode(void)
+{
+	x86_platform.realmode_init();
 	return 0;
 }
-early_initcall(init_real_mode);
+early_initcall(do_init_real_mode);

arch/x86/xen/enlighten_pv.c

@@ -1266,6 +1266,8 @@ asmlinkage __visible void __init xen_start_kernel(struct start_info *si)
 	xen_vcpu_info_reset(0);
 
 	x86_platform.get_nmi_reason = xen_get_nmi_reason;
+	x86_platform.realmode_reserve = x86_init_noop;
+	x86_platform.realmode_init = x86_init_noop;
 
 	x86_init.resources.memory_setup = xen_memory_setup;
 	x86_init.irqs.intr_mode_select	= x86_init_noop;