netdev - Re: [PATCH v2 2/5] VSOCK: support fill data to mergeable rx buffer in host

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Date:   Thu, 13 Dec 2018 15:42:33 +0800
From:   jiangyiwen <jiangyiwen@...wei.com>
To:     David Miller <davem@...emloft.net>
CC:     <stefanha@...hat.com>, <mst@...hat.com>, <jasowang@...hat.com>,
        <netdev@...r.kernel.org>, <kvm@...r.kernel.org>,
        <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH v2 2/5] VSOCK: support fill data to mergeable rx buffer in
 host

On 2018/12/13 13:59, David Miller wrote:
> From: jiangyiwen <jiangyiwen@...wei.com>
> Date: Thu, 13 Dec 2018 11:11:48 +0800
> 
>> I hope Host can fill fewer bytes into rx virtqueue, so
>> I keep structure virtio_vsock_mrg_rxbuf_hdr one byte
>> alignment.
> 
> The question is if this actully matters.
> 
> Do you know?
> 
> If the obejct this is embeeded inside of is at least 2 byte aligned,
> you are marking it packed for nothing.
> 
> There are only %100 downsides to using the packed attribute.
> 
> Simply define your datastructures properly, with fixed sized types,
> and all padding defined explicitly.
> 
> .
> 

Hi David,

Thanks a lot, I need to send number buffers from Host to Guest, so I think
we need to keep the structure size the same between host and guest.
But after your reminder, I feel my code may exist a serious problem,
that in mergeable mode, I send the total structure virtio_vsock_pkt
from Host to Guest, however, this structure size may be different
under different compilers (Guest and Host are different). Then, Guest
may parse the wrong packet length.

David, I want to ask if there is such a problem?

In addition, why I send total virtio_vsock_pkt structure from Host to Guest?
- In order to avoid to allocate virtio_vsock_pkt memory when receiving
  packets, in case of insufficient memory, it may have some advantages, and
  we may keep consistent with old version.

Thanks again,
Yiwen.