Linux Virtualization - Oct 2015 - [PATCH] vhost: fix performance on LE hosts

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 at linux.vnet.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst at redhat.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)
-- 
MST

On Tue, 27 Oct 2015 11:37:39 +0200
"Michael S. Tsirkin" <mst at redhat.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 at linux.vnet.ibm.com>
> Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
Oops my bad for this oversight...

Reviewed-by: Greg Kurz <gkurz at linux.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)

From: "Michael S. Tsirkin" <mst at redhat.com>
Date: Tue, 27 Oct 2015 11:37:39 +0200
> 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 at linux.vnet.ibm.com>
> Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
Michael, do you want me to take this via the 'net' tree?

On Tue, Oct 27, 2015 at 06:32:55AM -0700, David Miller
wrote:
> From: "Michael S. Tsirkin" <mst at redhat.com>
> Date: Tue, 27 Oct 2015 11:37:39 +0200
> 
> > 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 at linux.vnet.ibm.com>
> > Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
> 
> Michael, do you want me to take this via the 'net' tree?
Sure - especially since it looks like I have nothing else for this merge
window (working on fixing shutdown races, but it's slow going).  Thanks
a lot!

-- 
MST

From: "Michael S. Tsirkin" <mst at redhat.com>
Date: Tue, 27 Oct 2015 11:37:39 +0200
> 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 at linux.vnet.ibm.com>
> Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
Applied.