[PATCH 1/3] x86/ucode: Fix error handling during parallel ucode load

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

wait_for_state() returns false on encountering LOADING_EXIT.
control_thread_fn() can move directly to this state in the case of an early
error.  It is not an error condition for APs, but right now the latest write
into stopmachine_data.fn_result wins, causing the real error, -EIO, to get
clobbered with -EBUSY.  e.g.:

  # xen-ucode /lib/firmware/amd-ucode/microcode_amd_fam17h.bin --force
  Failed to update microcode. (err: Device or resource busy)

  (XEN) 256 cores are to update their microcode
  (XEN) microcode: CPU0 update rev 0x830107d to 0x830107c failed, result 
0x830107d
  (XEN) Late loading aborted: CPU0 failed to update ucode: -5

Drop all the -EBUSY's, and treat hitting LOADING_EXIT as a success case.  This
causes only a single error to be returned through stop_machine_run().  e.g.:

  # xen-ucode /lib/firmware/amd-ucode/microcode_amd_fam17h.bin --force
  Failed to update microcode. (err: Input/output error)

  (XEN) 256 cores are to update their microcode
  (XEN) microcode: CPU0 update rev 0x830107d to 0x830107c failed, result 
0x830107d
  (XEN) Late loading aborted: CPU0 failed to update ucode: -5

Fixes: 5ed12565aa32 ("microcode: rendezvous CPUs in NMI handler and load ucode")
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Xen 4.19 and earlier hit this case naturally, without the need of --force.
---
 xen/arch/x86/cpu/microcode/core.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/cpu/microcode/core.c 
b/xen/arch/x86/cpu/microcode/core.c
index 1d1a5aa4b097..ecee400f28a6 100644
--- a/xen/arch/x86/cpu/microcode/core.c
+++ b/xen/arch/x86/cpu/microcode/core.c
@@ -260,7 +260,9 @@ static int secondary_nmi_work(void)
 {
     cpumask_set_cpu(smp_processor_id(), &cpu_callin_map);
 
-    return wait_for_state(LOADING_EXIT) ? 0 : -EBUSY;
+    wait_for_state(LOADING_EXIT);
+
+    return 0;
 }
 
 static int primary_thread_work(const struct microcode_patch *patch,
@@ -271,7 +273,7 @@ static int primary_thread_work(const struct microcode_patch 
*patch,
     cpumask_set_cpu(smp_processor_id(), &cpu_callin_map);
 
     if ( !wait_for_state(LOADING_ENTER) )
-        return -EBUSY;
+        return 0;
 
     ret = alternative_call(ucode_ops.apply_microcode, patch, flags);
     if ( !ret )
@@ -313,7 +315,7 @@ static int cf_check microcode_nmi_callback(
 static int secondary_thread_fn(void)
 {
     if ( !wait_for_state(LOADING_CALLIN) )
-        return -EBUSY;
+        return 0;
 
     self_nmi();
 
@@ -336,7 +338,7 @@ static int primary_thread_fn(const struct microcode_patch 
*patch,
                              unsigned int flags)
 {
     if ( !wait_for_state(LOADING_CALLIN) )
-        return -EBUSY;
+        return 0;
 
     if ( ucode_in_nmi )
     {
-- 
2.39.5