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

From: Stefan Hajnoczi <stefanha@redhat.com>
To: jiangyiwen <jiangyiwen@huawei.com>
Cc: kvm@vger.kernel.org, mst@redhat.com, netdev@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	David Miller <davem@davemloft.net>
Subject: Re: [PATCH v2 2/5] VSOCK: support fill data to mergeable rx buffer in host
Date: Thu, 13 Dec 2018 15:46:16 +0000	[thread overview]
Message-ID: <20181213154616.GM23318__30347.2854699523$1544715881$gmane$org@stefanha-x1.localdomain> (raw)
In-Reply-To: <5C120D69.10201@huawei.com>

[-- Attachment #1.1: Type: text/plain, Size: 1792 bytes --]

On Thu, Dec 13, 2018 at 03:42:33PM +0800, jiangyiwen wrote:
> On 2018/12/13 13:59, David Miller wrote:
> > From: jiangyiwen <jiangyiwen@huawei.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.

Yes, virtio_vsock_pkt is internal driver state and should not be part of
the host<->guest interface (also for security reasons it's not good to
expose internal state structs across the interface).

Stefan

[-- Attachment #1.2: signature.asc --]
[-- Type: application/pgp-signature, Size: 455 bytes --]

[-- Attachment #2: Type: text/plain, Size: 183 bytes --]

_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization