netdev - Re: [PATCH] vhost: move is_le setup to the backend

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Message-Id: <20151102.150922.1956383631313633658.davem@davemloft.net>
Date:	Mon, 02 Nov 2015 15:09:22 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	gkurz@...ux.vnet.ibm.com
Cc:	mst@...hat.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] vhost: move is_le setup to the backend

From: Greg Kurz <gkurz@...ux.vnet.ibm.com>
Date: Fri, 30 Oct 2015 12:42:35 +0100

> The vq->is_le field is used to fix endianness when accessing the vring via
> the cpu_to_vhost16() and vhost16_to_cpu() helpers in the following cases:
> 
> 1) host is big endian and device is modern virtio
> 
> 2) host has cross-endian support and device is legacy virtio with a different
>    endianness than the host
> 
> Both cases rely on the VHOST_SET_FEATURES ioctl, but 2) also needs the
> VHOST_SET_VRING_ENDIAN ioctl to be called by userspace. Since vq->is_le
> is only needed when the backend is active, it was decided to set it at
> backend start.
> 
> This is currently done in vhost_init_used()->vhost_init_is_le() but it
> obfuscates the core vhost code. This patch moves the is_le setup to a
> dedicated function that is called from the backend code.
> 
> Note vhost_net is the only backend that can pass vq->private_data == NULL to
> vhost_init_used(), hence the "if (sock)" branch.
> 
> No behaviour change.
> 
> Signed-off-by: Greg Kurz <gkurz@...ux.vnet.ibm.com>

Michael, I'm assuming that you will be the one taking this.
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