[PATCH] xen x86: fix early boot crash with gcc-10

["Frédéric Pierret (fepitre)" <frederic.pierret@qubes-os.org>]

The change fixes boot failure on VM where kernel (at least v5.4 and v5.6)
is built with gcc-10 and STACKPROTECTOR_STRONG enabled:

```
Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: cpu_bringup_and_idle+0x93/0xa0
CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.4.31-1.qubes.x86_64 #1
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.12.0-1 04/01/2014
Call Trace:
  dump_stack+0x64/0x88
   panic+0x10b/0x2ed
   ? cpu_bringup_and_idle+0x93/0xa0
   __stack_chk_fail+0x15/0x20
   cpu_bringup_and_idle+0x93/0xa
```
The change makes successfully booting the VM. The VM is hosted by
KVM hypervisor and is running Xen into.

Based on work done by Sergei Trofimovich: https://lkml.org/lkml/2020/3/26/1133

Signed-off-by: Frédéric Pierret (fepitre) <frederic.pierret@qubes-os.org>
---
 arch/x86/xen/smp_pv.c          | 2 +-
 include/linux/compiler-gcc.h   | 1 +
 include/linux/compiler_types.h | 4 ++++
 3 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/xen/smp_pv.c b/arch/x86/xen/smp_pv.c
index 8fb8a50a28b4..5c8ee4a5bb0c 100644
--- a/arch/x86/xen/smp_pv.c
+++ b/arch/x86/xen/smp_pv.c
@@ -88,7 +88,7 @@ static void cpu_bringup(void)
 	local_irq_enable();
 }
 
-asmlinkage __visible void cpu_bringup_and_idle(void)
+asmlinkage __visible void __no_stack_protector cpu_bringup_and_idle(void)
 {
 	cpu_bringup();
 	boot_init_stack_canary();
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index d7ee4c6bad48..fb67c743138c 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -172,3 +172,4 @@
 #endif
 
 #define __no_fgcse __attribute__((optimize("-fno-gcse")))
+#define __no_stack_protector __attribute__((optimize("-fno-stack-protector")))
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index e970f97a7fcb..069c981eddb0 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -203,6 +203,10 @@ struct ftrace_likely_data {
 #define asm_inline asm
 #endif
 
+#ifndef __no_stack_protector
+# define __no_stack_protector
+#endif
+
 #ifndef __no_fgcse
 # define __no_fgcse
 #endif
-- 
2.25.1