Linux Virtualization - Mar 2013 - [RFC virt-spec PATCH] only writing out the last byte of MAC makes it have effect

The lengcy guests don't have mac programming command, we don't know when
it's safe to use MAC. We can change QEMU to make MAC change effect when
the last byte of MAC is written to config space.

Signed-off-by: Amos Kong <akong at redhat.com>
---
 virtio-spec.lyx | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/virtio-spec.lyx b/virtio-spec.lyx
index dbc4ef0..bb289fb 100644
--- a/virtio-spec.lyx
+++ b/virtio-spec.lyx
@@ -5430,7 +5430,7 @@ T_CTRL_MAC_TABLE_SET.
 
 \begin_layout Standard
 
-\change_inserted -1930653948 1358506710
+\change_inserted -1930653948 1363832689
 The config space 
 \begin_inset Quotes eld
 \end_inset
@@ -5464,6 +5464,15 @@ mac
  Therefore, VIRTIO_NET_CTRL_MAC_ADDR_SET is preferred, especially while
  the NIC is up.
  The command-specific-data is a 6-byte MAC address.
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted -1930653948 1363833477
+The legacy guests don't support the new command, they still change MAC
address
+ in original way, that's not atomic.
+ For more robust, QEMU only makes the MAC change effect when the last byte
+ of MAC address is written to config space.
 \change_unchanged
 
 \end_layout
-- 
1.8.1.4

The lengcy guests don't have mac programming command, we don't know when
it's safe to use MAC. This patch changed qemu to makes MAC change effect
when the last byte of MAC is written to config space.

MAC address takes first 6 bytes of config space of virtio-net, the addr
is 5 when the last byte is written in virtio_config_writeb().

MAC change will effect when n->mac is updated in virtio_net_set_config().

Signed-off-by: Amos Kong <akong at redhat.com>
---
 hw/virtio.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/hw/virtio.c b/hw/virtio.c
index 26fbc79..2e08302 100644
--- a/hw/virtio.c
+++ b/hw/virtio.c
@@ -605,8 +605,9 @@ void virtio_config_writeb(VirtIODevice *vdev, uint32_t addr,
uint32_t data)
 
     stb_p(vdev->config + addr, val);
 
-    if (vdev->set_config)
+    if (vdev->set_config && (addr == 5 || strcmp(vdev->name,
"virtio-net"))) {
         vdev->set_config(vdev, vdev->config);
+    }
 }
 
 void virtio_config_writew(VirtIODevice *vdev, uint32_t addr, uint32_t data)
-- 
1.7.1

On Thu, Mar 21, 2013 at 11:02:36AM +0800, Amos Kong
wrote:
> The lengcy guests don't have mac programming command, we don't know
when
> it's safe to use MAC. We can change QEMU to make MAC change effect when
> the last byte of MAC is written to config space.
> 
> Signed-off-by: Amos Kong <akong at redhat.com>
> ---
>  virtio-spec.lyx | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/virtio-spec.lyx b/virtio-spec.lyx
> index dbc4ef0..bb289fb 100644
> --- a/virtio-spec.lyx
> +++ b/virtio-spec.lyx
> @@ -5430,7 +5430,7 @@ T_CTRL_MAC_TABLE_SET.
>  
>  \begin_layout Standard
>  
> -\change_inserted -1930653948 1358506710
> +\change_inserted -1930653948 1363832689
>  The config space 
>  \begin_inset Quotes eld
>  \end_inset
> @@ -5464,6 +5464,15 @@ mac
>   Therefore, VIRTIO_NET_CTRL_MAC_ADDR_SET is preferred, especially while
>   the NIC is up.
>   The command-specific-data is a 6-byte MAC address.
> +\end_layout
> +
> +\begin_layout Standard
> +
> +\change_inserted -1930653948 1363833477
> +The legacy guests don't support the new command, they still change MAC
address
> + in original way, that's not atomic.
> + For more robust, QEMU only makes the MAC change effect when the last byte
> + of MAC address is written to config space.
>  \change_unchanged
>  
>  \end_layout
The wording can be improved, but before that - Rusty, what
do you think about such a hack? Worth supporting or
let's just ask everyone to update drivers?
> -- 
> 1.8.1.4