[Xen-devel] [PATCH v3 3/4] xen/arm: disable the event optimization in the gic

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

Currently we have an optimization in the GIC driver that allows us not
to request maintenance interrupts for Xen events. The basic idea is that
every time we are about to inject a new interrupt or event into a guest,
we check whether we can remove some old event irqs from one or more LRs.
We can do this because we know when a guest finishes processing event
notifications: it sets the evtchn_upcall_pending bit to 0.

However it is unsafe: the guest resets evtchn_upcall_pending before
EOI'ing the event irq, therefore a small window of time exists when Xen
could jump in and remove the event irq from the LR register before the
guest has EOI'ed it.

This is not a good practice according to the GIC manual.
Remove the optimization for now.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
---
 xen/arch/arm/gic.c |   42 ------------------------------------------
 1 files changed, 0 insertions(+), 42 deletions(-)

diff --git a/xen/arch/arm/gic.c b/xen/arch/arm/gic.c
index 1b852c1..2a8d1f4 100644
--- a/xen/arch/arm/gic.c
+++ b/xen/arch/arm/gic.c
@@ -37,7 +37,6 @@
                                      + (GIC_CR_OFFSET & 0xfff)))
 #define GICH ((volatile uint32_t *) (FIXMAP_ADDR(FIXMAP_GICH)  \
                                      + (GIC_HR_OFFSET & 0xfff)))
-static void events_maintenance(struct vcpu *v);
 static void gic_restore_pending_irqs(struct vcpu *v);
 
 /* Global state */
@@ -48,7 +47,6 @@ static struct {
     unsigned int lines;
     unsigned int cpus;
     spinlock_t lock;
-    uint64_t event_mask;
     uint64_t lr_mask;
 } gic;
 
@@ -62,7 +60,6 @@ void gic_save_state(struct vcpu *v)
     for ( i=0; i<nr_lrs; i++)
         v->arch.gic_lr[i] = GICH[GICH_LR + i];
     v->arch.lr_mask = gic.lr_mask;
-    v->arch.event_mask = gic.event_mask;
     /* Disable until next VCPU scheduled */
     GICH[GICH_HCR] = 0;
     isb();
@@ -76,7 +73,6 @@ void gic_restore_state(struct vcpu *v)
         return;
 
     gic.lr_mask = v->arch.lr_mask;
-    gic.event_mask = v->arch.event_mask;
     for ( i=0; i<nr_lrs; i++)
         GICH[GICH_LR + i] = v->arch.gic_lr[i];
     GICH[GICH_HCR] = GICH_HCR_EN;
@@ -318,7 +314,6 @@ int __init gic_init(void)
     gic_hyp_init();
 
     gic.lr_mask = 0ULL;
-    gic.event_mask = 0ULL;
 
     spin_unlock(&gic.lock);
 
@@ -421,9 +416,6 @@ static inline void gic_set_lr(int lr, unsigned int 
virtual_irq,
 
     BUG_ON(lr > nr_lrs);
 
-    if (virtual_irq == VGIC_IRQ_EVTCHN_CALLBACK && nr_lrs > 1)
-        maintenance_int = 0;
-
     GICH[GICH_LR + lr] = state |
         maintenance_int |
         ((priority >> 3) << GICH_LR_PRIORITY_SHIFT) |
@@ -436,11 +428,6 @@ void gic_set_guest_irq(struct vcpu *v, unsigned int 
virtual_irq,
     int i;
     struct pending_irq *iter, *n;
 
-    if ( v->is_running )
-    {
-        events_maintenance(v);
-    }
-
     spin_lock_irq(&gic.lock);
 
     if ( v->is_running && list_empty(&v->arch.vgic.lr_pending) )
@@ -448,8 +435,6 @@ void gic_set_guest_irq(struct vcpu *v, unsigned int 
virtual_irq,
         i = find_first_zero_bit(&gic.lr_mask, nr_lrs);
         if (i < nr_lrs) {
             set_bit(i, &gic.lr_mask);
-            if ( virtual_irq == VGIC_IRQ_EVTCHN_CALLBACK )
-                set_bit(i, &gic.event_mask);
             gic_set_lr(i, virtual_irq, state, priority);
             goto out;
         }
@@ -487,8 +472,6 @@ static void gic_restore_pending_irqs(struct vcpu *v)
         gic_set_lr(i, p->irq, GICH_LR_PENDING, p->priority);
         list_del_init(&p->lr_queue);
         set_bit(i, &gic.lr_mask);
-        if ( p->irq == VGIC_IRQ_EVTCHN_CALLBACK )
-            set_bit(i, &gic.event_mask);
     }
 
 }
@@ -577,27 +560,6 @@ void gicv_setup(struct domain *d)
                         GIC_BASE_ADDRESS + GIC_VR_OFFSET);
 }
 
-static void events_maintenance(struct vcpu *v)
-{
-    int i = 0;
-    int already_pending = test_bit(0,
-            (unsigned long *)&vcpu_info(v, evtchn_upcall_pending));
-
-    if (!already_pending && gic.event_mask != 0) {
-        spin_lock_irq(&gic.lock);
-        while ((i = find_next_bit((const long unsigned int *) &gic.event_mask,
-                        64, i)) < 64) {
-
-            GICH[GICH_LR + i] = 0;
-            clear_bit(i, &gic.lr_mask);
-            clear_bit(i, &gic.event_mask);
-
-            i++;
-        }
-        spin_unlock_irq(&gic.lock);
-    }
-}
-
 static void maintenance_interrupt(int irq, void *dev_id, struct cpu_user_regs 
*regs)
 {
     int i = 0, virq;
@@ -605,8 +567,6 @@ static void maintenance_interrupt(int irq, void *dev_id, 
struct cpu_user_regs *r
     struct vcpu *v = current;
     uint64_t eisr = GICH[GICH_EISR0] | (((uint64_t) GICH[GICH_EISR1]) << 32);
 
-    events_maintenance(v);
-
     while ((i = find_next_bit((const long unsigned int *) &eisr,
                               64, i)) < 64) {
         struct pending_irq *p;
@@ -622,8 +582,6 @@ static void maintenance_interrupt(int irq, void *dev_id, 
struct cpu_user_regs *r
             gic_set_lr(i, p->irq, GICH_LR_PENDING, p->priority);
             list_del_init(&p->lr_queue);
             set_bit(i, &gic.lr_mask);
-            if ( p->irq == VGIC_IRQ_EVTCHN_CALLBACK )
-                set_bit(i, &gic.event_mask);
         } else {
             gic_inject_irq_stop();
         }
-- 
1.7.2.5


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