[xen staging] x86/vioapic: top word redir entry writes don't trigger interrupts

[patchbot@xxxxxxx]

commit 6f59dc12962be61ef84774d0630bdb7c1050851f
Author:     Roger Pau Monné <roger.pau@xxxxxxxxxx>
AuthorDate: Thu Apr 1 16:41:48 2021 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Apr 1 16:41:48 2021 +0200

    x86/vioapic: top word redir entry writes don't trigger interrupts
    
    Top word writes just update the destination of the interrupt, but
    since there's no change on the masking or the triggering mode no
    guest interrupt injection can result of such write. Add an assert to
    that effect.
    
    Requested-by: Jan Beulich <jbeulich@xxxxxxxx>
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/hvm/vioapic.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/xen/arch/x86/hvm/vioapic.c b/xen/arch/x86/hvm/vioapic.c
index 804bc77279..e3ee747b7d 100644
--- a/xen/arch/x86/hvm/vioapic.c
+++ b/xen/arch/x86/hvm/vioapic.c
@@ -262,6 +262,8 @@ static void vioapic_write_redirent(
               !ent.fields.remote_irr &&
               hvm_irq->gsi_assert_count[gsi] )
     {
+        /* A top word write should never trigger an interrupt injection. */
+        ASSERT(!top_word);
         pent->fields.remote_irr = 1;
         vioapic_deliver(vioapic, idx);
     }
--
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