netdev - Re: [PATCH 2/3] vsockmon: Add vsockmon device.

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Date:   Mon, 22 Aug 2016 12:12:29 +0200
From:   Gerard Garcia <ggarcia@...a.uab.cat>
To:     David Miller <davem@...emloft.net>, David.Laight@...LAB.COM
Cc:     stefanha@...hat.com, netdev@...r.kernel.org, jhansen@...are.com,
        mst@...hat.com
Subject: Re: [PATCH 2/3] vsockmon: Add vsockmon device.

On 08/15/2016 07:23 PM, David Miller wrote:
> From: David Laight <David.Laight@...LAB.COM>
> Date: Mon, 15 Aug 2016 16:38:36 +0000
>
>> From: Stefan Hajnoczi
>>> Sent: 10 August 2016 12:52
>>> On Mon, Aug 08, 2016 at 06:14:41PM +0200, ggarcia@...a.uab.cat wrote:
>>>> diff --git a/include/uapi/linux/vsockmon.h b/include/uapi/linux/vsockmon.h
>>>> new file mode 100644
>>>> index 0000000..739b4bf
>>>> --- /dev/null
>>>> +++ b/include/uapi/linux/vsockmon.h
>>>> @@ -0,0 +1,35 @@
>>>> +#ifndef _UAPI_VSOCKMON_H
>>>> +#define _UAPI_VSOCKMON_H
>>>> +
>>>> +#include <linux/virtio_vsock.h>
>>>> +
>>>> +/* Structure of packets received trought the vsockmon device. */
>>>> +
>>>> +struct af_vsockmon_hdr {
>>>> +	__le64 src_cid;
>>>> +	__le64 dst_cid;
>>>> +	__le32 src_port;
>>>> +	__le32 dst_port;
>>>> +	__le16 op;			/* enum af_vsockmon_op */
>>>> +	__le16 t;			/* enum af_vosckmon_t */
>>>> +	__le16 len;			/* sizeof(t_hdr) */
>>>> +	union {
>>>> +		struct virtio_vsock_hdr virtio_hdr;
>>>> +	} t_hdr;
>>>> +} __attribute__((packed));
>> ...
>>
>> Gah, another 'packed' structure.
>> Have you looked at the amount of code the sparc64 compiler generates
>> to access the structure members??
>>
>> You really want to add another 16bit field and enforce 64bit alignment
>> on the header and all data blocks.
>
> Indeed, avoid the packed attribute at all costs.
>

I understand. I'll add another 16b field so it is aligned and avoid the 
packed attribute.

Gerard