[PATCH] kvm: x86: Fix racy event propagation in kmv timer

[PATCH] kvm: x86: Fix racy event propagation in kmv timer

[Jan Kiszka]

Minor issues that likely had no practical relevance: The kvm timer
function so far incremented the pending counter and then may reset it
again to 1 in case reinjection was disabled. This opened a small racy
window with the corresponding VCPU loop that may have happened to run on
another (real) CPU and already consumed the value.

Moveover, the previous code tried to optimize the setting of
KVM_REQ_PENDING_TIMER, but used atomic_inc_and_test - which always
returns true unless pending had the invalid value of -1 on entry.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
---

 arch/x86/kvm/timer.c |   11 +++++------
 1 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/timer.c b/arch/x86/kvm/timer.c
index 86dbac0..18fd17c 100644
--- a/arch/x86/kvm/timer.c
+++ b/arch/x86/kvm/timer.c
@@ -9,12 +9,11 @@ static int __kvm_timer_fn(struct kvm_vcpu *vcpu, struct kvm_timer *ktimer)
 	int restart_timer = 0;
 	wait_queue_head_t *q = &vcpu->wq;
 
-	/* FIXME: this code should not know anything about vcpus */
-	if (!atomic_inc_and_test(&ktimer->pending))
-		set_bit(KVM_REQ_PENDING_TIMER, &vcpu->requests);
-
-	if (!ktimer->reinject)
-		atomic_set(&ktimer->pending, 1);
+	if (ktimer->reinject || !atomic_read(&ktimer->pending)) {
+		/* FIXME: this code should not know anything about vcpus */
+		if (atomic_inc_return(&ktimer->pending) == 1)
+			set_bit(KVM_REQ_PENDING_TIMER, &vcpu->requests);
+	}
 
 	if (waitqueue_active(q))
 		wake_up_interruptible(q);

[PATCH v2 1/2] kvm: x86: Fix racy event propagation in kmv timer

[Jan Kiszka]

v2 as requested in private discussion: Broken into two pieces, and the
second one will not change the original semantic.

-------------->

Minor issue that likely had no practical relevance: The kvm timer
function so far incremented the pending counter and then may reset it
again to 1 in case reinjection was disabled. This opened a small racy
window with the corresponding VCPU loop that may have happened to run on
another (real) CPU and already consumed the value.

Fix it by skipping the incrementation in case pending is already > 0.
This opens a different race windows, but may only rarely cause lost
events in case we do not care about them anyway (!reinject).

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
---

 arch/x86/kvm/timer.c |   16 ++++++++++------
 1 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/timer.c b/arch/x86/kvm/timer.c
index 86dbac0..36054e9 100644
--- a/arch/x86/kvm/timer.c
+++ b/arch/x86/kvm/timer.c
@@ -9,12 +9,16 @@ static int __kvm_timer_fn(struct kvm_vcpu *vcpu, struct kvm_timer *ktimer)
 	int restart_timer = 0;
 	wait_queue_head_t *q = &vcpu->wq;
 
-	/* FIXME: this code should not know anything about vcpus */
-	if (!atomic_inc_and_test(&ktimer->pending))
-		set_bit(KVM_REQ_PENDING_TIMER, &vcpu->requests);
-
-	if (!ktimer->reinject)
-		atomic_set(&ktimer->pending, 1);
+	/*
+	 * There is a race window between reading and incrementing, but we do
+	 * not care about potentially loosing timer events in the !reinject
+	 * case anyway.
+	 */
+	if (ktimer->reinject || !atomic_read(&ktimer->pending)) {
+		/* FIXME: this code should not know anything about vcpus */
+		if (!atomic_inc_and_test(&ktimer->pending))
+			set_bit(KVM_REQ_PENDING_TIMER, &vcpu->requests);
+	}
 
 	if (waitqueue_active(q))
 		wake_up_interruptible(q);

Re: [PATCH v2 1/2] kvm: x86: Fix racy event propagation in kmv timer

[Marcelo Tosatti]

On Tue, Jun 09, 2009 at 03:37:01PM +0200, Jan Kiszka wrote:
> v2 as requested in private discussion: Broken into two pieces, and the
> second one will not change the original semantic.
> 
> -------------->
> 
> Minor issue that likely had no practical relevance: The kvm timer
> function so far incremented the pending counter and then may reset it
> again to 1 in case reinjection was disabled. This opened a small racy
> window with the corresponding VCPU loop that may have happened to run on
> another (real) CPU and already consumed the value.
> 
> Fix it by skipping the incrementation in case pending is already > 0.
> This opens a different race windows, but may only rarely cause lost
> events in case we do not care about them anyway (!reinject).
> 
> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

ACK both, thanks.

Re: [PATCH v2 1/2] kvm: x86: Fix racy event propagation in kmv timer

[Avi Kivity]

Marcelo Tosatti wrote:
> On Tue, Jun 09, 2009 at 03:37:01PM +0200, Jan Kiszka wrote:
>   
>> v2 as requested in private discussion: Broken into two pieces, and the
>> second one will not change the original semantic.
>>
>> -------------->
>>
>> Minor issue that likely had no practical relevance: The kvm timer
>> function so far incremented the pending counter and then may reset it
>> again to 1 in case reinjection was disabled. This opened a small racy
>> window with the corresponding VCPU loop that may have happened to run on
>> another (real) CPU and already consumed the value.
>>
>> Fix it by skipping the incrementation in case pending is already > 0.
>> This opens a different race windows, but may only rarely cause lost
>> events in case we do not care about them anyway (!reinject).
>>
>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>>     
>
> ACK both, thanks.
>   

And applied both, thanks.

-- 
error compiling committee.c: too many arguments to function

[PATCH v2 2/2] kvm: x86: Drop useless atomic test from timer function

[Jan Kiszka]

The current code tries to optimize the setting of KVM_REQ_PENDING_TIMER
but used atomic_inc_and_test - which always returns true unless pending
had the invalid value of -1 on entry. This patch drops the test part
preserving the original semantic but expressing it less confusingly.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
---

 arch/x86/kvm/timer.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/timer.c b/arch/x86/kvm/timer.c
index 36054e9..8dd1a55 100644
--- a/arch/x86/kvm/timer.c
+++ b/arch/x86/kvm/timer.c
@@ -15,9 +15,9 @@ static int __kvm_timer_fn(struct kvm_vcpu *vcpu, struct kvm_timer *ktimer)
 	 * case anyway.
 	 */
 	if (ktimer->reinject || !atomic_read(&ktimer->pending)) {
+		atomic_inc(&ktimer->pending);
 		/* FIXME: this code should not know anything about vcpus */
-		if (!atomic_inc_and_test(&ktimer->pending))
-			set_bit(KVM_REQ_PENDING_TIMER, &vcpu->requests);
+		set_bit(KVM_REQ_PENDING_TIMER, &vcpu->requests);
 	}
 
 	if (waitqueue_active(q))