Re: [Qemu-devel] [PATCH] char: report errors from qio_channel_{read, write}v_full

[Greg Edwards]

On Fri, Sep 29, 2017 at 11:12:41AM +0200, Paolo Bonzini wrote:
> On 28/09/2017 22:52, Greg Edwards wrote:
>> Two callers of qio_channel_{read,write}v_full were not passing in an
>> Error pointer, missing any error messages from the channel class
>> io_{read,write}v methods.
>
> This is on purpose in order to avoid "spamming" the logs.  In
> particular, for sockets it can be a normal thing for the other side to
> disconnect.

Thanks for the background, Paolo.

We had encountered a vhost-user-scsi initialization failure, and were
looking for the errno from the recvmsg failure from
VHOST_USER_GET_FEATURES:

qemu-system-x86_64: -device vhost-user-scsi-pci,chardev=vus0,bootindex=2: 
Failed to read msg header. Read -1 instead of 12. Original request 1.
qemu-system-x86_64: -device vhost-user-scsi-pci,chardev=vus0,bootindex=2: 
vhost-user-scsi: vhost initialization failed: Operation not permitted

In this case, the strerror in vhost_user_scsi_realize is just reporting
on the -1 returned from the vhost_user_read failure, not the errno of
original offender.  That's what started me down this path.

Greg