[xen staging] x86/guest: move allocation of Xen upcall vector to init code

[patchbot@xxxxxxx]

commit 731c47684ff7de39405c2ed3affc5f043225e543
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Mon Nov 24 11:25:08 2025 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Nov 24 11:25:08 2025 +0100

    x86/guest: move allocation of Xen upcall vector to init code
    
    There's no need to do this every time init_evtchn() is called. Just do it
    once when setting up CPU0. Drop the assertion as well, as
    alloc_hipriority_vector() (called by alloc_direct_apic_vector()) uses more
    restrictive BUG_ON() anyway. Then evtchn_upcall_vector can also validly
    become ro-after-init, just that it needs to move out of init_evtchn().
    
    While touching the function, add a comment why other seemingly-BSP code
    can't move out of there (into e.g. an __init one).
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/guest/xen/xen.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/guest/xen/xen.c b/xen/arch/x86/guest/xen/xen.c
index 5c9f393c75..77a3a8742a 100644
--- a/xen/arch/x86/guest/xen/xen.c
+++ b/xen/arch/x86/guest/xen/xen.c
@@ -233,16 +233,12 @@ static void cf_check xen_evtchn_upcall(void)
     ack_APIC_irq();
 }
 
+static uint8_t __ro_after_init evtchn_upcall_vector;
+
 static int init_evtchn(void)
 {
-    static uint8_t evtchn_upcall_vector;
     int rc;
 
-    if ( !evtchn_upcall_vector )
-        alloc_direct_apic_vector(&evtchn_upcall_vector, xen_evtchn_upcall);
-
-    ASSERT(evtchn_upcall_vector);
-
     rc = xen_hypercall_set_evtchn_upcall_vector(this_cpu(vcpu_id),
                                                 evtchn_upcall_vector);
     if ( rc )
@@ -251,6 +247,10 @@ static int init_evtchn(void)
         return rc;
     }
 
+    /*
+     * HVM_PARAM_CALLBACK_IRQ is not moved on migrate, so has to be set up
+     * again on resume.
+     */
     if ( smp_processor_id() == 0 )
     {
         struct xen_hvm_param a = {
@@ -293,6 +293,8 @@ static void __init cf_check setup(void)
                XEN_LEGACY_MAX_VCPUS);
     }
 
+    alloc_direct_apic_vector(&evtchn_upcall_vector, xen_evtchn_upcall);
+
     BUG_ON(init_evtchn());
 }
 
--
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