From 488f069bd1f3aeb6ee748acb02b7581831bcb3f8 Mon Sep 17 00:00:00 2001 From: "Michael S. Tsirkin" Date: Thu, 11 Jul 2013 16:06:46 +0300 Subject: virtio-net: add feature bit for any header s/g Old qemu versions required that 1st s/g entry is the header. Since QEMU 1.5, patchset titled "virtio-net: iovec handling cleanup" removed this limitation but a feature bit is needed so guests know it's safe to lay out header differently. This patch applies on top and adds such a feature bit to QEMU. It is set by default for virtio-net. virtio net header inline with the data is beneficial for latency and small packet bandwidth - guest driver code utilizing this feature has been acked but missed 3.11 by a narrow margin, it's pending for 3.12. This feature bit is cleared by default when compatibility with old machine types is requested. Other performance-sensitive devices (blk and scsi) don't yet support arbitrary s/g layouts, so we only set this bit for virtio-net for now. There are plans to allow arbitrary layouts there, but no code has been posted yet. Cc: Rusty Russell Reviewed-by: Laszlo Ersek Signed-off-by: Michael S. Tsirkin --- include/hw/virtio/virtio.h | 2 ++ 1 file changed, 2 insertions(+) (limited to 'include/hw/virtio/virtio.h') diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h index a6c5c5380c..5d1d2be295 100644 --- a/include/hw/virtio/virtio.h +++ b/include/hw/virtio/virtio.h @@ -43,6 +43,8 @@ /* We notify when the ring is completely used, even if the guest is suppressing * callbacks */ #define VIRTIO_F_NOTIFY_ON_EMPTY 24 +/* Can the device handle any descriptor layout? */ +#define VIRTIO_F_ANY_LAYOUT 27 /* We support indirect buffer descriptors */ #define VIRTIO_RING_F_INDIRECT_DESC 28 /* The Guest publishes the used index for which it expects an interrupt -- cgit 1.4.1