[xen staging] x86/dpci: do not leak pending interrupts on CPU offline

[patchbot@xxxxxxx]

commit 29555668b5725b9d5393b72bfe7ff9a3fa606714
Author:     Roger Pau Monné <roger.pau@xxxxxxxxxx>
AuthorDate: Mon Oct 7 11:10:21 2024 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Oct 7 11:10:21 2024 +0200

    x86/dpci: do not leak pending interrupts on CPU offline
    
    The current dpci logic relies on a softirq being executed as a side effect 
of
    the cpu_notifier_call_chain() call in the code path that offlines the target
    CPU.  However the call to cpu_notifier_call_chain() won't trigger any 
softirq
    processing, and even if it did, such processing should be done after all
    interrupts have been migrated off the current CPU, otherwise new pending 
dpci
    interrupts could still appear.
    
    Currently the ASSERT() in the cpu callback notifier is fairly easy to 
trigger
    by doing CPU offline from a PVH dom0.
    
    Solve this by instead moving out any dpci interrupts pending processing once
    the CPU is dead.  This might introduce more latency than attempting to drain
    before the CPU is put offline, but it's less complex, and CPU 
online/offline is
    not a common action.  Any extra introduced latency should be tolerable.
    
    Fixes: f6dd295381f4 ('dpci: replace tasklet with softirq')
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/drivers/passthrough/x86/hvm.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/xen/drivers/passthrough/x86/hvm.c 
b/xen/drivers/passthrough/x86/hvm.c
index d3627e4af7..f5faff7a49 100644
--- a/xen/drivers/passthrough/x86/hvm.c
+++ b/xen/drivers/passthrough/x86/hvm.c
@@ -1105,23 +1105,27 @@ static int cf_check cpu_callback(
     struct notifier_block *nfb, unsigned long action, void *hcpu)
 {
     unsigned int cpu = (unsigned long)hcpu;
+    unsigned long flags;
 
     switch ( action )
     {
     case CPU_UP_PREPARE:
         INIT_LIST_HEAD(&per_cpu(dpci_list, cpu));
         break;
+
     case CPU_UP_CANCELED:
-    case CPU_DEAD:
-        /*
-         * On CPU_DYING this callback is called (on the CPU that is dying)
-         * with an possible HVM_DPIC_SOFTIRQ pending - at which point we can
-         * clear out any outstanding domains (by the virtue of the idle loop
-         * calling the softirq later). In CPU_DEAD case the CPU is deaf and
-         * there are no pending softirqs for us to handle so we can chill.
-         */
         ASSERT(list_empty(&per_cpu(dpci_list, cpu)));
         break;
+
+    case CPU_DEAD:
+        if ( list_empty(&per_cpu(dpci_list, cpu)) )
+            break;
+        /* Take whatever dpci interrupts are pending on the dead CPU. */
+        local_irq_save(flags);
+        list_splice_init(&per_cpu(dpci_list, cpu), &this_cpu(dpci_list));
+        local_irq_restore(flags);
+        raise_softirq(HVM_DPCI_SOFTIRQ);
+        break;
     }
 
     return NOTIFY_DONE;
--
generated by git-patchbot for /home/xen/git/xen.git#staging