[xen stable-4.20] x86/S3: don't lose previously collected CPU data

[patchbot@xxxxxxx]

commit a586e3865acf5ac1a085ccaa0b3ad284f25179f6
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Jul 22 09:54:05 2025 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Jul 22 09:54:05 2025 +0200

    x86/S3: don't lose previously collected CPU data
    
    smp_store_cpu_info() doesn't call identify_cpu() again during S3 resume,
    hence it is wrong to overwrite APs' data using boot_cpu_data. With
    7126b7f806d54 ("x86/CPU: re-work populating of cpu_data[]") this now
    results in a crash, as the cpu_to_socket() use in smp_store_cpu_info()
    then obtains a bad socket number, much like was already observed in [1].
    
    [1] https://lists.xen.org/archives/html/xen-devel/2025-06/msg01879.html
    
    Fixes: bb502a8ca5925 ("x86: check feature flags after resume")
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    master commit: acaa69d9859d3c4e3ee92fe4d48d292bda9568b9
    master date: 2025-07-17 12:49:18 +0200
---
 xen/arch/x86/smpboot.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c
index ab5947f1c1..10d57dfb82 100644
--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -366,7 +366,9 @@ void asmlinkage start_secondary(void *unused)
 
     cpu_init();
 
-    initialize_cpu_data(cpu);
+    /* During resume, must not clear previously collected data. */
+    if ( system_state != SYS_STATE_resume )
+        initialize_cpu_data(cpu);
 
     microcode_update_one();
 
--
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