linux-kernel - Re: [PATCH net-next V2] vhost: switch to use new message format

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Message-Id: <20180806.104148.1576759632957723909.davem@davemloft.net>
Date:   Mon, 06 Aug 2018 10:41:48 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     jasowang@...hat.com
Cc:     mst@...hat.com, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next V2] vhost: switch to use new message format

From: Jason Wang <jasowang@...hat.com>
Date: Mon,  6 Aug 2018 11:17:47 +0800

> We use to have message like:
> 
> struct vhost_msg {
> 	int type;
> 	union {
> 		struct vhost_iotlb_msg iotlb;
> 		__u8 padding[64];
> 	};
> };
> 
> Unfortunately, there will be a hole of 32bit in 64bit machine because
> of the alignment. This leads a different formats between 32bit API and
> 64bit API. What's more it will break 32bit program running on 64bit
> machine.
> 
> So fixing this by introducing a new message type with an explicit
> 32bit reserved field after type like:
> 
> struct vhost_msg_v2 {
> 	__u32 type;
> 	__u32 reserved;
> 	union {
> 		struct vhost_iotlb_msg iotlb;
> 		__u8 padding[64];
> 	};
> };
> 
> We will have a consistent ABI after switching to use this. To enable
> this capability, introduce a new ioctl (VHOST_SET_BAKCEND_FEATURE) for
> userspace to enable this feature (VHOST_BACKEND_F_IOTLB_V2).
> 
> Fixes: 6b1e6cc7855b ("vhost: new device IOTLB API")
> Signed-off-by: Jason Wang <jasowang@...hat.com>
> ---
> Changes from V1:
> - use __u32 instead of int for type

Applied, thanks Jason.