Hi Jason,
Thanks for your response.
On Thu, 18 Feb 2021 at 14:18, Jason Wang <jasowang@xxxxxxxxxx> wrote:
Hi Gautam:
On 2021/2/15 9:01 下午, Gautam Dawar wrote:
Hi Jason/Michael,
I observed a kernel panic while testing vhost-vdpa with Xilinx adapters. Here are the details for your review:
Problem statement:
When qemu with vhost-vdpa netdevice is run for the first time, it works well. But after the VM is powered off, next qemu run causes kernel panic due to a NULL pointer dereference in irq_bypass_register_producer().
Root cause analysis:
When the VM is powered off, vhost_dev_stop() is invoked which in turn calls vhost_vdpa_reset_device() causing the irq_bypass producers to be unregistered.
On the next run, when qemu opens the vhost device, the vhost_vdpa_open() file operation calls vhost_dev_init(). Here, call_ctx->producer memory is cleared in vhost_vring_call_reset().
Further, when the virtqueues are initialized by vhost_virtqueue_init(), vhost_vdpa_setup_vq_irq() again registers the irq_bypass producer for each virtqueue. As the node member of struct irq_bypass_producer is also initialized to zero, traversal on the producers list causes crash due to NULL pointer dereference.
Thanks a lot for reporting this issue.
Fix details:
I think that this issue can be fixed by invoking vhost_vdpa_setup_vq_irq() only when vhost_vdpa_set_status() includes VIRTIO_CONFIG_S_DRIVER_OK in the new status value. This way, there won’t be any stale nodes in the irqbypass module’s producers list which are reset in vhost_vring_call_reset().
Patch:
diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c index 62a9bb0efc55..fdad94e2fbf9 100644
--- a/drivers/vhost/vdpa.c
+++ b/drivers/vhost/vdpa.c
@@ -409,7 +409,6 @@ static long vhost_vdpa_vring_ioctl(struct vhost_vdpa *v, unsigned int cmd,
cb.private = NULL;
}
ops->set_vq_cb(vdpa, idx, &cb);
- vhost_vdpa_setup_vq_irq(v, idx);
break;
case VHOST_SET_VRING_NUM:
We can also track this issue in Bugzilla ticket 21171 (https://bugzilla.kernel.org/show_bug.cgi?id=211711) and the complete patch is attached with this email.
So vhost supports to remove or switch eventfd through vhost_vdpa_vring_ioctl(). So if userspace want to switch to another eventfd, we should re-do the register and unregister.
GD>> This makes sense. I missed the use case where userspace may want to switch to a different eventfd.
This can happen when interrupt needs to be disabled for some
reason (e.g MSI-X is masked).
I think we need to deal this issue in another way. Can we check whether or not the producer is initialized before?
Thanks
GD>> Initialization path is fine but the actual problem lies in the clean-up part.I think the following check is the cause of this issue:
static void vhost_vdpa_clean_irq(struct vhost_vdpa *v)if (vq->call_ctx.producer.irq)
irq_bypass_unregister_producer(&vq->call_ctx.producer);
The above if condition will prevent the de-initialization of the producer nodes corresponding to irq 0 but irq_bypass_unregister_producer() should be called for all valid irq values including zero.
Accordingly, following patch is required to fix this issue:
diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
index 62a9bb0efc55..d1c3a33c6239 100644
--- a/drivers/vhost/vdpa.c
+++ b/drivers/vhost/vdpa.c
@@ -849,7 +849,7 @@ static void vhost_vdpa_clean_irq(struct vhost_vdpa *v)
for (i = 0; i < v->nvqs; i++) {
vq = &v->vqs[i];
- if (vq->call_ctx.producer.irq)
+ if (vq->call_ctx.producer.irq >= 0)
irq_bypass_unregister_producer(&vq->call_ctx.producer);
}
}
It should work, please post a formal patch for this.
I will give more thought in the meanwhile since I spot some other defects on codes for irqbyass usage in vdpa.
Thanks
The revised patch (bug211711_fix.patch) is also attached with this email.
Regards,
Gautam Dawar
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