x86/cpu: revert opt_allow_unsafe from __ro_after_init to __read_mostly

Making opt_allow_unsafe read only after init requires changes to the logic in
init_amd(), otherwise the following #PF happens on CPU hotplug:

----[ Xen-4.20.0-1-d  x86_64  debug=y  Tainted:     H  ]----
CPU:    1
RIP:    e008:[<ffff82d040291081>] arch/x86/cpu/amd.c#init_amd+0x37f/0x993
[...]
Xen call trace:
   [<ffff82d040291081>] R arch/x86/cpu/amd.c#init_amd+0x37f/0x993
   [<ffff82d040291fbe>] F identify_cpu+0x2d4/0x4db
   [<ffff82d04032eeaa>] F start_secondary+0x22e/0x3cf
   [<ffff82d040203327>] F __high_start+0x87/0xa0

Pagetable walk from ffff82d0404011ea:
 L4[0x105] = 000000006fc2e063 ffffffffffffffff
 L3[0x141] = 000000006fc2b063 ffffffffffffffff
 L2[0x002] = 000000807c7ca063 ffffffffffffffff
 L1[0x001] = 800000006f801121 ffffffffffffffff

****************************************
Panic on CPU 1:
FATAL PAGE FAULT
[error_code=0003]
Faulting linear address: ffff82d0404011ea
****************************************

For the time being revert opt_allow_unsafe to be __read_mostly.

Fixes: bfcb0abb191f ('types: replace remaining uses of s8')
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

diff --git a/xen/arch/x86/cpu/amd.c b/xen/arch/x86/cpu/amd.c
index 903be14af4b9e3a6aa95c530fc0675cc4a277174..7da04230393a9f84cb2a5e42c385ae954742148c 100644 (file)
--- a/xen/arch/x86/cpu/amd.c
+++ b/xen/arch/x86/cpu/amd.c
@@ -46,7 +46,7 @@ static unsigned int __initdata opt_cpuid_mask_thermal_ecx = ~0u;
 integer_param("cpuid_mask_thermal_ecx", opt_cpuid_mask_thermal_ecx);
 
 /* 1 = allow, 0 = don't allow guest creation, -1 = don't allow boot */
-int8_t __ro_after_init opt_allow_unsafe;
+int8_t __read_mostly opt_allow_unsafe;
 boolean_param("allow_unsafe", opt_allow_unsafe);
 
 /* Signal whether the ACPI C1E quirk is required. */