linux-kernel - Re: [PATCH 3/3] virtio: fix endianness check for vhost support

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Date:	Wed, 2 Mar 2016 01:27:54 +0000
From:	Will Deacon <will.deacon@....com>
To:	Andre Przywara <andre.przywara@....com>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Pekka Enberg <penberg@...nel.org>, kvm@...r.kernel.org,
	kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] virtio: fix endianness check for vhost support

On Tue, Mar 01, 2016 at 04:49:38PM +0000, Andre Przywara wrote:
> Currently we deny any VHOST_* functionality if the architecture
> supports guests with different endianness than the host. Most of the
> time even on those architectures the endianness of guest and host are
> the same, though, so we are denying the glory of VHOST needlessly.
> Switch from compile time determination to a run time scheme, which
> takes the actual endianness of the guest into account.
> For this we change the semantics of VIRTIO_ENDIAN_HOST to return the
> actual endianness of the host (the endianness of kvmtool at compile
> time, really). The actual check in vhost_net now compares this against
> the guest endianness.
> This enables vhost support on ARM and ARM64.
> 
> Signed-off-by: Andre Przywara <andre.przywara@....com>
> ---
>  include/kvm/virtio.h | 9 +++++++--
>  virtio/net.c         | 2 +-
>  2 files changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/include/kvm/virtio.h b/include/kvm/virtio.h
> index 768ee96..66530fd 100644
> --- a/include/kvm/virtio.h
> +++ b/include/kvm/virtio.h
> @@ -17,10 +17,15 @@
>  #define VIRTIO_PCI_O_CONFIG	0
>  #define VIRTIO_PCI_O_MSIX	1
>  
> -#define VIRTIO_ENDIAN_HOST	0
>  #define VIRTIO_ENDIAN_LE	(1 << 0)
>  #define VIRTIO_ENDIAN_BE	(1 << 1)
>  
> +#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> +#define VIRTIO_ENDIAN_HOST VIRTIO_ENDIAN_LE
> +#else
> +#define VIRTIO_ENDIAN_HOST VIRTIO_ENDIAN_BE
> +#endif

pci.h already uses __BYTE_ORDER and __LITTLE_ENDIAN, so we should at least
be consistent here.

Will