[PATCH] x86/ucode: Exit early from early_update_cache() if loading not available

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

If for any reason early_microcode_init() concludes that no microcode loading
is available, early_update_cache() will fall over a NULL function pointer:

  (XEN) Xen call trace:
  (XEN)    [<ffff82d04037372e>] R show_code+0x91/0x18f
  (XEN)    [<ffff82d040373a49>] F show_execution_state+0x2d/0x1fc
  (XEN)    [<ffff82d040374210>] F fatal_trap+0x87/0x19a
  (XEN)    [<ffff82d040647f2c>] F init_idt_traps+0/0x1bd
  (XEN)    [<ffff82d04063854f>] F early_page_fault+0x8f/0x94
  (XEN)    [<0000000000000000>] F 0000000000000000
  (XEN)    [<ffff82d040628c46>] F 
arch/x86/cpu/microcode/core.c#early_update_cache+0x11/0x74
  (XEN)    [<ffff82d040628e5c>] F microcode_init_cache+0x5a/0x5c
  (XEN)    [<ffff82d04064388f>] F __start_xen+0x1e11/0x27ee
  (XEN)    [<ffff82d040206184>] F __high_start+0x94/0xa0

which is actually parse_blob()'s use of ucode_ops.collect_cpu_info.

Skip trying to cache anything if microcode loading is unavailable.

Fixes: dc380df12acf ("x86/ucode: load microcode earlier on boot CPU")
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>

Found while doing something unrelated, but this is going to interact poorly
with MCU_CONTROL_DIS_MCU_LOAD.
---
 xen/arch/x86/cpu/microcode/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/xen/arch/x86/cpu/microcode/core.c 
b/xen/arch/x86/cpu/microcode/core.c
index 5a5c0a8c70db..9029301107d6 100644
--- a/xen/arch/x86/cpu/microcode/core.c
+++ b/xen/arch/x86/cpu/microcode/core.c
@@ -789,6 +789,9 @@ int __init microcode_init_cache(unsigned long *module_map,
 {
     int rc = 0;
 
+    if ( !ucode_ops.apply_microcode )
+        return -ENODEV;
+
     if ( ucode_scan )
         /* Need to rescan the modules because they might have been relocated */
         microcode_scan_module(module_map, mbi);

base-commit: 59d0bf62861f5c9b317ccf89f8b5c8b4d19927ad
prerequisite-patch-id: c3f6ae7def85b63808449493e3b5185bc40c405d
prerequisite-patch-id: 59a20dfb4778c62bf512f746e36b1bea0949b0a8
prerequisite-patch-id: a70c8dd42245affe402b08cacd5872b5a32a6d69
prerequisite-patch-id: 3efc26e008858670286c173f77f8ec34ddfd9df1
prerequisite-patch-id: 5f6ddddf7dd6029f401d13bbb87ac3bb88c15700
prerequisite-patch-id: 4133b7d49c978a89042e95f899f46c4ec4ac4498
prerequisite-patch-id: d2d3a24a650f6b1b50e279be158cdd097eb43a4b
prerequisite-patch-id: 358299b6b56983e3c069ea1f30e7cf214b0a2c54
prerequisite-patch-id: b17530cf5672ada3e7792606b7a3bef55c8aa372
prerequisite-patch-id: e9bc40cc80e61b24d90eeb7097cd9b703f0170a6
-- 
2.30.2