[Xen-devel] [PATCH v10 24/32] ARM: GICv3: handle unmapped LPIs

[Andre Przywara <andre.przywara@xxxxxxx>]

When LPIs get unmapped by a guest, they might still be in some LR of
some VCPU. Nevertheless we remove the corresponding pending_irq
(possibly freeing it), and detect this case (irq_to_pending() returns
NULL) when the LR gets cleaned up later.
However a *new* LPI may get mapped with the same number while the old
LPI is *still* in some LR. To avoid getting the wrong state, we mark
every newly mapped LPI as PRISTINE, which means: has never been in an
LR before. If we detect the LPI in an LR anyway, it must have been an
older one, which we can simply retire.
Before inserting such a PRISTINE LPI into an LR, we must make sure that
it's not already in another LR, as the architecture forbids two
interrupts with the same virtual IRQ number on one CPU.

Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>
---
 xen/arch/arm/gic.c         | 51 ++++++++++++++++++++++++++++++++++++++++++----
 xen/include/asm-arm/vgic.h |  6 ++++++
 2 files changed, 53 insertions(+), 4 deletions(-)

diff --git a/xen/arch/arm/gic.c b/xen/arch/arm/gic.c
index 9e32103..da5ba86 100644
--- a/xen/arch/arm/gic.c
+++ b/xen/arch/arm/gic.c
@@ -375,6 +375,8 @@ static inline void gic_set_lr(int lr, struct pending_irq *p,
 {
     ASSERT(!local_irq_is_enabled());
 
+    clear_bit(GIC_IRQ_GUEST_PRISTINE_LPI, &p->status);
+
     gic_hw_ops->update_lr(lr, p, state);
 
     set_bit(GIC_IRQ_GUEST_VISIBLE, &p->status);
@@ -441,6 +443,40 @@ void gic_raise_inflight_irq(struct vcpu *v, unsigned int 
virtual_irq)
 #endif
 }
 
+/*
+ * Find an unused LR to insert an IRQ into, starting with the LR given
+ * by @lr. If this new interrupt is a PRISTINE LPI, scan the other LRs to
+ * avoid inserting the same IRQ twice. This situation can occur when an
+ * event gets discarded while the LPI is in an LR, and a new LPI with the
+ * same number gets mapped quickly afterwards.
+ */
+static unsigned int gic_find_unused_lr(struct vcpu *v,
+                                       struct pending_irq *p,
+                                       unsigned int lr)
+{
+    unsigned int nr_lrs = gic_hw_ops->info->nr_lrs;
+    unsigned long *lr_mask = (unsigned long *) &this_cpu(lr_mask);
+    struct gic_lr lr_val;
+
+    ASSERT(spin_is_locked(&v->arch.vgic.lock));
+
+    if ( test_bit(GIC_IRQ_GUEST_PRISTINE_LPI, &p->status) )
+    {
+        unsigned int used_lr = 0;
+
+        while ( (used_lr = find_next_bit(lr_mask, nr_lrs, used_lr)) < nr_lrs )
+        {
+            gic_hw_ops->read_lr(used_lr, &lr_val);
+            if ( lr_val.virq == p->irq )
+                return used_lr;
+        }
+    }
+
+    lr = find_next_zero_bit(lr_mask, nr_lrs, lr);
+
+    return lr;
+}
+
 void gic_raise_guest_irq(struct vcpu *v, unsigned int virtual_irq,
         unsigned int priority)
 {
@@ -456,7 +492,8 @@ void gic_raise_guest_irq(struct vcpu *v, unsigned int 
virtual_irq,
 
     if ( v == current && list_empty(&v->arch.vgic.lr_pending) )
     {
-        i = find_first_zero_bit(&this_cpu(lr_mask), nr_lrs);
+        i = gic_find_unused_lr(v, p, 0);
+
         if (i < nr_lrs) {
             set_bit(i, &this_cpu(lr_mask));
             gic_set_lr(i, p, GICH_LR_PENDING);
@@ -479,8 +516,14 @@ static void gic_update_one_lr(struct vcpu *v, int i)
     gic_hw_ops->read_lr(i, &lr_val);
     irq = lr_val.virq;
     p = irq_to_pending(v, irq);
-    /* An LPI might have been unmapped, in which case we just clean up here. */
-    if ( unlikely(!p) )
+    /*
+     * An LPI might have been unmapped, in which case we just clean up here.
+     * If that LPI is marked as PRISTINE, the information in the LR is bogus,
+     * as it belongs to a previous, already unmapped LPI. So we discard it
+     * here as well.
+     */
+    if ( unlikely(!p) ||
+         test_and_clear_bit(GIC_IRQ_GUEST_PRISTINE_LPI, &p->status) )
     {
         ASSERT(is_lpi(irq));
 
@@ -590,7 +633,7 @@ static void gic_restore_pending_irqs(struct vcpu *v)
     inflight_r = &v->arch.vgic.inflight_irqs;
     list_for_each_entry_safe ( p, t, &v->arch.vgic.lr_pending, lr_queue )
     {
-        lr = find_next_zero_bit(&this_cpu(lr_mask), nr_lrs, lr);
+        lr = gic_find_unused_lr(v, p, lr);
         if ( lr >= nr_lrs )
         {
             /* No more free LRs: find a lower priority irq to evict */
diff --git a/xen/include/asm-arm/vgic.h b/xen/include/asm-arm/vgic.h
index 02732db..3fc4ceb 100644
--- a/xen/include/asm-arm/vgic.h
+++ b/xen/include/asm-arm/vgic.h
@@ -60,12 +60,18 @@ struct pending_irq
      * vcpu while it is still inflight and on an GICH_LR register on the
      * old vcpu.
      *
+     * GIC_IRQ_GUEST_PRISTINE_LPI: the IRQ is a newly mapped LPI, which
+     * has never been in an LR before. This means that any trace of an
+     * LPI with the same number in an LR must be from an older LPI, which
+     * has been unmapped before.
+     *
      */
 #define GIC_IRQ_GUEST_QUEUED   0
 #define GIC_IRQ_GUEST_ACTIVE   1
 #define GIC_IRQ_GUEST_VISIBLE  2
 #define GIC_IRQ_GUEST_ENABLED  3
 #define GIC_IRQ_GUEST_MIGRATING   4
+#define GIC_IRQ_GUEST_PRISTINE_LPI  5
     unsigned long status;
     struct irq_desc *desc; /* only set it the irq corresponds to a physical 
irq */
     unsigned int irq;
-- 
2.9.0


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