[PATCH v7 0/2] xen/arm: remove race conditions in irq migration

[PATCH v7 0/2] xen/arm: remove race conditions in irq migration

[Stefano Stabellini]

Hi all,

this patch series removes three race conditions affecting the current
code base.

The first race condition is between gic_update_one_lr and
vgic_vcpu_inject_irq: as soon as gic_update_one_lr calls
irq_set_affinity a new interrupt could be injected in the new pcpu,
eventually vgic_vcpu_inject_irq is called which manipulates the inflight
list. The first patch solves this race by adding a barrier in
gic_update_one_lr. This patch was suggested by Julien.

The second race condition happens when gic_update_one_lr runs
simultaneously with vgic_store_itargetsr and vgic_migrate_irq. Setting
the new target is done after calling vgic_migrate_irq, which means that
gic_update_one_lr could end up setting the physical affinity to the one
of the old pcpu.

The third race condition happens again between gic_update_one_lr and
vgic_migrate_irq: when GIC_IRQ_GUEST_MIGRATING is already set and
vgic_migrate_irq is called again, it will take a different vgic lock
from the one that gic_update_one_lr is taking.

The second patch addressed the last two issues by refusing any irq
migration requests while one request is already in-progress and not yet
completed.


For your reference, it is not possible to take the p->desc lock from
gic_update_one_lr, because the correct lock ordering is p->desc lock,
then vgic lock.


Changes in v7:
- add reviewed-by
- code style fix
- s/%d/%u
- gdprintk/gprintk

Changes in v6:
- smp_mb/smb_wmb
- refuse nested irq migration requests instead of trying to handle them


Stefano Stabellini (2):
      arm: remove irq from inflight, then change physical affinity
      vgic: refuse irq migration when one is already in progress

 xen/arch/arm/gic.c         | 10 +++++++++-
 xen/arch/arm/vgic-v2.c     |  7 +++----
 xen/arch/arm/vgic-v3.c     |  7 ++++---
 xen/arch/arm/vgic.c        | 14 +++++++++-----
 xen/include/asm-arm/vgic.h |  2 +-
 5 files changed, 26 insertions(+), 14 deletions(-)

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[PATCH v7 1/2] arm: remove irq from inflight, then change physical affinity

[Stefano Stabellini]

This patch fixes a potential race that could happen when
gic_update_one_lr and vgic_vcpu_inject_irq run simultaneously.

When GIC_IRQ_GUEST_MIGRATING is set, we must make sure that the irq has
been removed from inflight before changing physical affinity, to avoid
concurrent accesses to p->inflight, as vgic_vcpu_inject_irq will take a
different vcpu lock.

Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Julien Grall <julien.grall@arm.com>
---
 xen/arch/arm/gic.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/xen/arch/arm/gic.c b/xen/arch/arm/gic.c
index 9522c6c..2455d8f 100644
--- a/xen/arch/arm/gic.c
+++ b/xen/arch/arm/gic.c
@@ -503,6 +503,13 @@ static void gic_update_one_lr(struct vcpu *v, int i)
             gic_raise_guest_irq(v, irq, p->priority);
         else {
             list_del_init(&p->inflight);
+            /* 
+             * Remove from inflight, then change physical affinity. It
+             * makes sure that when a new interrupt is received on the
+             * next pcpu, inflight is already cleared. No concurrent
+             * accesses to inflight.
+             */
+            smp_wmb();
             if ( test_and_clear_bit(GIC_IRQ_GUEST_MIGRATING, &p->status) )
             {
                 struct vcpu *v_target = vgic_get_target_vcpu(v, irq);
-- 
1.9.1


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[PATCH v7 2/2] vgic: refuse irq migration when one is already in progress

[Stefano Stabellini]

When an irq migration is already in progress, but not yet completed
(GIC_IRQ_GUEST_MIGRATING is set), refuse any other irq migration
requests for the same irq.

This patch implements this approach by returning success or failure from
vgic_migrate_irq, and avoiding irq target changes on failure. It prints
a warning in case the irq migration fails.

It also moves the clear_bit of GIC_IRQ_GUEST_MIGRATING to after the
physical irq affinity has been changed so that all operations regarding
irq migration are completed.

Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
---
 xen/arch/arm/gic.c         |  3 ++-
 xen/arch/arm/vgic-v2.c     |  7 +++----
 xen/arch/arm/vgic-v3.c     |  7 ++++---
 xen/arch/arm/vgic.c        | 14 +++++++++-----
 xen/include/asm-arm/vgic.h |  2 +-
 5 files changed, 19 insertions(+), 14 deletions(-)

diff --git a/xen/arch/arm/gic.c b/xen/arch/arm/gic.c
index 2455d8f..f943931 100644
--- a/xen/arch/arm/gic.c
+++ b/xen/arch/arm/gic.c
@@ -510,10 +510,11 @@ static void gic_update_one_lr(struct vcpu *v, int i)
              * accesses to inflight.
              */
             smp_wmb();
-            if ( test_and_clear_bit(GIC_IRQ_GUEST_MIGRATING, &p->status) )
+            if ( test_bit(GIC_IRQ_GUEST_MIGRATING, &p->status) )
             {
                 struct vcpu *v_target = vgic_get_target_vcpu(v, irq);
                 irq_set_affinity(p->desc, cpumask_of(v_target->processor));
+                clear_bit(GIC_IRQ_GUEST_MIGRATING, &p->status);
             }
         }
     }
diff --git a/xen/arch/arm/vgic-v2.c b/xen/arch/arm/vgic-v2.c
index 0674f7b..dc9f95b 100644
--- a/xen/arch/arm/vgic-v2.c
+++ b/xen/arch/arm/vgic-v2.c
@@ -156,12 +156,11 @@ static void vgic_store_itargetsr(struct domain *d, struct vgic_irq_rank *rank,
         /* Only migrate the vIRQ if the target vCPU has changed */
         if ( new_target != old_target )
         {
-            vgic_migrate_irq(d->vcpu[old_target],
+            if ( vgic_migrate_irq(d->vcpu[old_target],
                              d->vcpu[new_target],
-                             virq);
+                             virq) )
+                write_atomic(&rank->vcpu[offset], new_target);
         }
-
-        write_atomic(&rank->vcpu[offset], new_target);
     }
 }
 
diff --git a/xen/arch/arm/vgic-v3.c b/xen/arch/arm/vgic-v3.c
index 0679e76..1e9890b 100644
--- a/xen/arch/arm/vgic-v3.c
+++ b/xen/arch/arm/vgic-v3.c
@@ -151,9 +151,10 @@ static void vgic_store_irouter(struct domain *d, struct vgic_irq_rank *rank,
 
     /* Only migrate the IRQ if the target vCPU has changed */
     if ( new_vcpu != old_vcpu )
-        vgic_migrate_irq(old_vcpu, new_vcpu, virq);
-
-    write_atomic(&rank->vcpu[offset], new_vcpu->vcpu_id);
+    {
+        if ( vgic_migrate_irq(old_vcpu, new_vcpu, virq) )
+            write_atomic(&rank->vcpu[offset], new_vcpu->vcpu_id);
+    }
 }
 
 static inline bool vgic_reg64_check_access(struct hsr_dabt dabt)
diff --git a/xen/arch/arm/vgic.c b/xen/arch/arm/vgic.c
index 67d75a6..83569b0 100644
--- a/xen/arch/arm/vgic.c
+++ b/xen/arch/arm/vgic.c
@@ -237,18 +237,21 @@ static int vgic_get_virq_priority(struct vcpu *v, unsigned int virq)
     return priority;
 }
 
-void vgic_migrate_irq(struct vcpu *old, struct vcpu *new, unsigned int irq)
+bool vgic_migrate_irq(struct vcpu *old, struct vcpu *new, unsigned int irq)
 {
     unsigned long flags;
     struct pending_irq *p = irq_to_pending(old, irq);
 
     /* nothing to do for virtual interrupts */
     if ( p->desc == NULL )
-        return;
+        return true;
 
     /* migration already in progress, no need to do anything */
     if ( test_bit(GIC_IRQ_GUEST_MIGRATING, &p->status) )
-        return;
+    {
+        gprintk(XENLOG_WARNING, "irq %u migration failed: requested while in progress\n", irq);
+        return false;
+    }
 
     perfc_incr(vgic_irq_migrates);
 
@@ -258,7 +261,7 @@ void vgic_migrate_irq(struct vcpu *old, struct vcpu *new, unsigned int irq)
     {
         irq_set_affinity(p->desc, cpumask_of(new->processor));
         spin_unlock_irqrestore(&old->arch.vgic.lock, flags);
-        return;
+        return true;
     }
     /* If the IRQ is still lr_pending, re-inject it to the new vcpu */
     if ( !list_empty(&p->lr_queue) )
@@ -269,7 +272,7 @@ void vgic_migrate_irq(struct vcpu *old, struct vcpu *new, unsigned int irq)
         irq_set_affinity(p->desc, cpumask_of(new->processor));
         spin_unlock_irqrestore(&old->arch.vgic.lock, flags);
         vgic_vcpu_inject_irq(new, irq);
-        return;
+        return true;
     }
     /* if the IRQ is in a GICH_LR register, set GIC_IRQ_GUEST_MIGRATING
      * and wait for the EOI */
@@ -277,6 +280,7 @@ void vgic_migrate_irq(struct vcpu *old, struct vcpu *new, unsigned int irq)
         set_bit(GIC_IRQ_GUEST_MIGRATING, &p->status);
 
     spin_unlock_irqrestore(&old->arch.vgic.lock, flags);
+    return true;
 }
 
 void arch_move_irqs(struct vcpu *v)
diff --git a/xen/include/asm-arm/vgic.h b/xen/include/asm-arm/vgic.h
index 894c3f1..544867a 100644
--- a/xen/include/asm-arm/vgic.h
+++ b/xen/include/asm-arm/vgic.h
@@ -314,7 +314,7 @@ extern int vcpu_vgic_free(struct vcpu *v);
 extern bool vgic_to_sgi(struct vcpu *v, register_t sgir,
                         enum gic_sgi_mode irqmode, int virq,
                         const struct sgi_target *target);
-extern void vgic_migrate_irq(struct vcpu *old, struct vcpu *new, unsigned int irq);
+extern bool vgic_migrate_irq(struct vcpu *old, struct vcpu *new, unsigned int irq);
 
 /* Reserve a specific guest vIRQ */
 extern bool vgic_reserve_virq(struct domain *d, unsigned int virq);
-- 
1.9.1


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Re: [PATCH v7 2/2] vgic: refuse irq migration when one is already in progress

[Julien Grall]

Hi Stefano,

On 04/05/2017 09:28 PM, Stefano Stabellini wrote:
> When an irq migration is already in progress, but not yet completed
> (GIC_IRQ_GUEST_MIGRATING is set), refuse any other irq migration
> requests for the same irq.
>
> This patch implements this approach by returning success or failure from
> vgic_migrate_irq, and avoiding irq target changes on failure. It prints
> a warning in case the irq migration fails.
>
> It also moves the clear_bit of GIC_IRQ_GUEST_MIGRATING to after the
> physical irq affinity has been changed so that all operations regarding
> irq migration are completed.
>
> Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>

Reviewed-by: Julien Grall <julien.grall@arm.com>

Cheers,

-- 
Julien Grall

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