linux-kernel - Re: [PATCH] vhost: fix performance on LE hosts

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Message-ID: <20151027113018.3b40bd78@bahia.local>
Date:	Tue, 27 Oct 2015 11:30:18 +0100
From:	Greg Kurz <gkurz@...ux.vnet.ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH] vhost: fix performance on LE hosts

On Tue, 27 Oct 2015 11:37:39 +0200
"Michael S. Tsirkin" <mst@...hat.com> wrote:
> commit 2751c9882b947292fcfb084c4f604e01724af804 ("vhost: cross-endian
> support for legacy devices") introduced a minor regression: even with
> cross-endian disabled, and even on LE host, vhost_is_little_endian is
> checking is_le flag so there's always a branch.
> 
> To fix, simply check virtio_legacy_is_little_endian first.
> 
> Cc: Greg Kurz <gkurz@...ux.vnet.ibm.com>
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>

Oops my bad for this oversight...

Reviewed-by: Greg Kurz <gkurz@...ux.vnet.ibm.com>

> ---
>  drivers/vhost/vhost.h | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 4772862..d3f7674 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -183,10 +183,17 @@ static inline bool vhost_has_feature(struct vhost_virtqueue *vq, int bit)
>  	return vq->acked_features & (1ULL << bit);
>  }
> 
> +#ifdef CONFIG_VHOST_CROSS_ENDIAN_LEGACY
>  static inline bool vhost_is_little_endian(struct vhost_virtqueue *vq)
>  {
>  	return vq->is_le;
>  }
> +#else
> +static inline bool vhost_is_little_endian(struct vhost_virtqueue *vq)
> +{
> +	return virtio_legacy_is_little_endian() || vq->is_le;
> +}
> +#endif
> 
>  /* Memory accessors */
>  static inline u16 vhost16_to_cpu(struct vhost_virtqueue *vq, __virtio16 val)

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