linux-kernel - Re: [PATCH v37 0/3] Virtio-balloon: support free page reporting

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Date:   Thu, 27 Dec 2018 13:17:20 +0100
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Wei Wang <wei.w.wang@...el.com>, virtio-dev@...ts.oasis-open.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, mst@...hat.com, mhocko@...nel.org,
        akpm@...ux-foundation.org, dgilbert@...hat.com
Cc:     torvalds@...ux-foundation.org, pbonzini@...hat.com,
        liliang.opensource@...il.com, yang.zhang.wz@...il.com,
        quan.xu0@...il.com, nilal@...hat.com, riel@...hat.com,
        peterx@...hat.com, quintela@...hat.com,
        Halil Pasic <pasic@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>
Subject: Re: [PATCH v37 0/3] Virtio-balloon: support free page reporting



On 27.12.2018 12:59, Christian Borntraeger wrote:
> On 27.12.2018 12:31, Christian Borntraeger wrote:
>> This patch triggers random crashes in the guest kernel on s390 early during boot.
>> No migration and no setting of the balloon is involved.
>>
> 
> Adding Conny and Halil,
> 
> As the QEMU provides no PAGE_HINT feature yet, this quick hack makes the
> guest boot fine again:
> 
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 728ecd1eea305..aa2e1864c5736 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -492,7 +492,7 @@ static int init_vqs(struct virtio_balloon *vb)
>                 callbacks[VIRTIO_BALLOON_VQ_FREE_PAGE] = NULL;
>         }
>  
> -       err = vb->vdev->config->find_vqs(vb->vdev, VIRTIO_BALLOON_VQ_MAX,
> +       err = vb->vdev->config->find_vqs(vb->vdev, 3, //VIRTIO_BALLOON_VQ_MAX,
>                                          vqs, callbacks, names, NULL, NULL);
>         if (err)
>                 return err;
> 
> 
> To me it looks like that virtio_ccw_find_vqs will abort if any of the virtqueues 
> that it is been asked for does not exist (including the earlier ones).
> 

This "hack" makes the random crashes go away, but the balloon interface itself
does not work. (setting the value to anything will hang the guest). 
As patch 1 also modifies the main path, there seem to be additional issues, maybe
endianess

Looking at things like

+		vb->cmd_id_received = VIRTIO_BALLOON_CMD_ID_STOP;
+		vb->cmd_id_active = cpu_to_virtio32(vb->vdev,
+						  VIRTIO_BALLOON_CMD_ID_STOP);
+		vb->cmd_id_stop = cpu_to_virtio32(vb->vdev,
+						  VIRTIO_BALLOON_CMD_ID_STOP);


Why is cmd_id_received not using cpu_to_virtio32?