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 <rusty@rustcorp.com.au>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

include/hw/i386/pc.h

@@ -229,6 +229,10 @@ int e820_add_entry(uint64_t, uint64_t, uint32_t);
             .driver   = "Nehalem-" TYPE_X86_CPU,\
             .property = "level",\
             .value    = stringify(2),\
+        },{\
+            .driver   = "virtio-net-pci",\
+            .property = "any_layout",\
+            .value    = "off",\
         }
 
 #define PC_COMPAT_1_4 \

include/hw/virtio/virtio-net.h

@@ -243,6 +243,7 @@ struct virtio_net_ctrl_mq {
 
 #define DEFINE_VIRTIO_NET_FEATURES(_state, _field) \
         DEFINE_VIRTIO_COMMON_FEATURES(_state, _field), \
+        DEFINE_PROP_BIT("any_layout", _state, _field, VIRTIO_F_ANY_LAYOUT, true), \
         DEFINE_PROP_BIT("csum", _state, _field, VIRTIO_NET_F_CSUM, true), \
         DEFINE_PROP_BIT("guest_csum", _state, _field, VIRTIO_NET_F_GUEST_CSUM, true), \
         DEFINE_PROP_BIT("gso", _state, _field, VIRTIO_NET_F_GSO, true), \

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