Michael,
Thank you very much for your help. I greatly appreciate it.
Steven
On 12/17/18, 4:16 PM, "Michael S. Tsirkin" <m...@redhat.com> wrote:
On Mon, Dec 17, 2018 at 11:56:59PM +0000, Steven Luong (sluong) wrote:
>
>
> On 12/17/18, 2:55 PM, "Michael S. Tsirkin" <m...@redhat.com> wrote:
>
> On Thu, Dec 13, 2018 at 11:24:28PM +0000, Steven Luong (sluong) via
Virtualization wrote:
> > Folks,
> >
> >
> >
> > We came across a memory race condition between VPP vhost driver and
the kernel
> > vhost. VPP is running a tap interface over vhost backend. In this
case, VPP is
> > acting as the vhost driver mode and the kernel vhost is acting as
the vhost
> > device mode.
> >
> >
> >
> > In the kernel vhost’s TX traffic direction which is VPP’s RX
traffic direction,
> > kernel vhost is the producer and VPP is the consumer.
>
> Looking at vhost net kernel code, it seems to use the
> same terminology as virtio net.
> Can you pls clarify which ring number do you use 0 or 1?
>
> <SVL> 0. </SVL>
>
> I will assume ring 0 below.
>
>
>
> > Kernel vhost places
> > traffic in kernel vhost’s TX vring. VPP removes traffic in VPP’s RX
vring.
>
>
> So VPP makes buffers available and vhost kernel uses them?
>
> <SVL> Correct. </SVL>
>
> > It
> > is inevitable that the vring may become full under heavy traffic
and the kernel
> > vhost may have to stop and wait for the guest (VPP) to empty the
vring and to
> > refill the vring with descriptors. When that case happens, kernel
vhost clears
> > the bit in the vring’s used flag to request an
interrupt/notification. Due to
> > shared memory race condition, VPP may miss the clearing of the
vring’s used
> > flag from kernel vhost and didn’t send kernel vhost an interrupt
after VPP
> > empties and refills the vring with new descriptors.
>
> So kernel vhost's wakeups are signalled through eventfd - I assume
> this is what you mean by an interrupt?
>
>
> To prevent the race that you describe, vhost has this code:
>
> /* OK, now we need to know about added descriptors. */
> if (!headcount) {
> if (unlikely(busyloop_intr)) {
> vhost_poll_queue(&vq->poll);
> } else if
(unlikely(vhost_enable_notify(&net->dev, vq))) {
> /* They have slipped one in as we were
> * doing that: check again. */
> vhost_disable_notify(&net->dev, vq);
> continue;
> }
> /* Nothing new? Wait for eventfd to tell us
>
> So ring should be re-checked after notifications are enabled.
> If ring is no longer empty, vhost will proceed to handle the ring.
> This code has been around for many years.
>
> Thus if VPP doesn't work with it then I suspect it does something
> differently, e.g. is it possible that it is missing a memory
> barrier after writing out the available buffers and before
> checking the flags?
>
> <SVL> Can the race condition be avoided totally? Here is the scenario.
>
> t0: VPP refills the vring with new descriptors, not yet sets avail->idx
to make it available to kernel vhost.
> t1: kernel vhost checks the vring, still sees no space available, but
does not yet execute the line of code to clear the used->flags
> t2: vpp executes sfence, and sets avail->idx to make it available to
kernel vhost
At this point you need a full memory barrier. E.g. mfence, or xor.
Otherwise the read can get re-ordered before the write.
> t3: vpp checks used->flags, it is still 1, vpp skips sending the interrupt
> t4: kernel vhost clears used->flags to indicate interrupt is required.
But VPP just missed it.
fine but at this point kernel vhost will recheck using vhost_get_avail
and process the buffers. So the lack of notification isn't a problem:
bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
{
__virtio16 avail_idx;
int r;
if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
return false;
vq->used_flags &= ~VRING_USED_F_NO_NOTIFY;
if (!vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
r = vhost_update_used_flags(vq);
if (r) {
vq_err(vq, "Failed to enable notification at %p:
%d\n",
&vq->used->flags, r);
return false;
}
} else {
r = vhost_update_avail_event(vq, vq->avail_idx);
if (r) {
vq_err(vq, "Failed to update avail event index at
%p: %d\n",
vhost_avail_event(vq), r);
return false;
}
}
/* They could have slipped one in as we were doing that: make
* sure it's written, then check again. */
smp_mb();
r = vhost_get_avail(vq, avail_idx, &vq->avail->idx);
if (r) {
vq_err(vq, "Failed to check avail idx at %p: %d\n",
&vq->avail->idx, r);
return false;
}
return vhost16_to_cpu(vq, avail_idx) != vq->avail_idx;
}
>The result is kernel vhost got stuck even though the vring is filled with
new descriptors.
Shouldn't be. Maybe you are missing a fence before reading used->flags?
> Steven
>
> </SVL>
>
>
> > Unfortunately, this missed
> > notification causes the kernel vhost to be stuck because once the
kernel vhost
> > is waiting for an interrupt/notification from the guest, only an
interrupt/
> > notification from the guest can resume/re-enable the guest from
submitting new
> > packets to the vring. This design seems vulnerable.
>
> The protocol right now relies on notifications never being lost.
>
> I can imagine an alternative where device also re-checks
> the rings periodically, but that would involve running
> timers host side which is only possible if there's a
> free processor that can handle them. Further,
> it will still lead to stalls and packet drops, which will
> be harder to debug.
>
> That's just my $.02, pls feel free to take it with the virtio tc
> maybe others will feel differently.
>
> > Should the kernel vhost
> > totally depend on the notification from the guest? Quoting the text
from
> >
> >
> >
> > http://docs.oasis-open.org/virtio/virtio/v1.0/virtio-v1.0.html
> >
> >
> >
> > /* The device uses this in used->flags to advise the driver: don’t
kick me
> > * when you add a buffer. It’s unreliable, so it’s simply an
> > * optimization. */
> > #define VIRTQ_USED_F_NO_NOTIFY 1
> >
> >
> >
> > I interpret that the notification is simply an optimization, not a
reliable
> > notification mechanism.
>
> What was meant I think is that suppression is unreliable.
>
> >So the kernel vhost should not bet the farm on it.
> >
> >
> >
> > We encounter the same race condition in kernel vhost’s RX traffic
direction
> > which causes the kernel vhost to be stuck due to missed
interrupt/notification
> > although it is less frequent.
>
>
> For the reverse direction the spec actually has some pseudo-code and
a suggestion
> about handling that race:
>
> For optimal performance, a driver MAY disable used buffer
notifications while processing the used
> ring, but beware the problem of missing notifications between
emptying the ring and reenabling no-
> tifications. This is usually handled by re-checking for more
used buffers after notifications are re-
> enabled:
>
> virtq_disable_used_buffer_notifications(vq);
> for (;;) {
> if (vq->last_seen_used !=
le16_to_cpu(virtq->used.idx)) {
> virtq_enable_used_buffer_notifications(vq);
> mb();
> if (vq->last_seen_used !=
le16_to_cpu(virtq->used.idx))
> break;
> virtq_disable_used_buffer_notifications(vq);
> }
> struct virtq_used_elem *e =
virtq.used->ring[vq->last_seen_used%vsz];
> process_buffer(e);
> vq->last_seen_used++;
>
>
>
> >
> >
> > Steven
> >
>
> > _______________________________________________
> > Virtualization mailing list
> > Virtualization@lists.linux-foundation.org
> > https://lists.linuxfoundation.org/mailman/listinfo/virtualization
>
>
>
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