Subject: Re: vhost changes (batched) in linux-next after 12/13 trigger random crashes in KVM guests after reboot

From: Christian Borntraeger <[email-masked]>
Date: Thu, 13 Feb 2020 10:30:03 +0100

On 12.02.20 17:34, Eugenio Pérez wrote:
> On Tue, 2020-02-11 at 14:13 +0100, Christian Borntraeger wrote:
>>
>> On 11.02.20 14:04, Eugenio Pérez wrote:
>>> On Mon, 2020-02-10 at 12:01 +0100, Christian Borntraeger wrote:
>>>> On 10.02.20 10:47, Eugenio Perez Martin wrote:
>>>>> Hi Christian.
>>>>>
>>>>> I'm not able to reproduce the failure with eccb852f1fe6bede630e2e4f1a121a81e34354ab commit. Could you add more
>>>>> data?
>>>>> Your configuration (libvirt or qemu line), and host's dmesg output if any?
>>>>>
>>>>> Thanks!
>>>>
>>>> If it was not obvious, this is on s390x, a big endian system.
>>>>
>>>
>>> Hi Christian. Thank you very much for your fast responses.
>>>
>>> Could you try this patch on top of eccb852f1fe6bede630e2e4f1a121a81e34354ab?
>>
>> I still get 
>> [   43.665145] Guest moved used index from 0 to 289
>> after some reboots.
>>
>>
>>> Thanks!
>>>
>>> From 71d0f9108a18aa894cc0c0c1c7efbad39f465a27 Mon Sep 17 00:00:00 2001
>>> From: =?UTF-8?q?Eugenio=20P=C3=A9rez?= <
>>> [email-masked]>
>>> Date: Tue, 11 Feb 2020 13:19:10 +0100
>>> Subject: [PATCH] vhost: fix return value of vhost_get_vq_desc
>>>
>>> Before of the batch change, it was the chain's head. Need to keep that
>>> way or we will not be able to free a chain of descriptors.
>>>
>>> Fixes: eccb852f1fe6 ("vhost: batching fetches")
>>> ---
>>>  drivers/vhost/vhost.c | 3 +--
>>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>>> index b5a51b1f2e79..fc422c3e5c08 100644
>>> --- a/drivers/vhost/vhost.c
>>> +++ b/drivers/vhost/vhost.c
>>> @@ -2409,12 +2409,11 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
>>>  			*out_num += ret;
>>>  		}
>>>  
>>> -		ret = desc->id;
>>> -
>>>  		if (!(desc->flags & VRING_DESC_F_NEXT))
>>>  			break;
>>>  	}
>>>  
>>> +	ret = vq->descs[vq->first_desc].id;
>>>  	vq->first_desc = i + 1;
>>>  
>>>  	return ret;
>>>
> 
> Sorry, still not able to reproduce the issue.
> 
> Could we try to disable all the vhost features?
> 
> Thanks!
> 
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 661088ae6dc7..08f6d2ccb697 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -250,11 +250,11 @@ int vhost_init_device_iotlb(struct vhost_dev *d, bool enabled);
>         } while (0)
>  
>  enum {
> -       VHOST_FEATURES = (1ULL << VIRTIO_F_NOTIFY_ON_EMPTY) |
> -                        (1ULL << VIRTIO_RING_F_INDIRECT_DESC) |
> -                        (1ULL << VIRTIO_RING_F_EVENT_IDX) |
> -                        (1ULL << VHOST_F_LOG_ALL) |
> -                        (1ULL << VIRTIO_F_ANY_LAYOUT) |
> +       VHOST_FEATURES = /* (1ULL << VIRTIO_F_NOTIFY_ON_EMPTY) | */
> +                        /* (1ULL << VIRTIO_RING_F_INDIRECT_DESC) | */
> +                        /* (1ULL << VIRTIO_RING_F_EVENT_IDX) | */
> +                        /* (1ULL << VHOST_F_LOG_ALL) | */
> +                        /* (1ULL << VIRTIO_F_ANY_LAYOUT) | */
>                          (1ULL << VIRTIO_F_VERSION_1)
>  };
> 

I still get  guest crashes with this on top of eccb852f1fe6. (The patch did not
apply, I had to manually comment out these things)