Re: [PATCH RFC 2/2] virtio_net: enable probing for NEEDS_RESET support

Started by Michael S. Tsirkin on Aug 29, 2017 · 2 messages from 2 authors · Last reply Aug 29, 2017

open the first message on its own page

Michael S. Tsirkin

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On Tue, Aug 29, 2017 at 04:07:59PM -0400, Willem de Bruijn wrote:
From: Willem de Bruijn <willemb@google.com>

Implement a mechanism to signal that a virtio device implements the
VIRTIO_CONFIG_S_NEEDS_RESET command.

Testing for VIRTIO_CONFIG_S_NEEDS_RESET support by issuing the request
and verifying the reset state would require an expensive state change.

To avoid that, add a status bit to the feature register used during
feature negotiation between host and guest.

Set the bit for virtio-net, which supports the feature.

Signed-off-by: Willem de Bruijn <willemb@google.com>
All virtio 1 devices have the reset feature so maybe guest does
not need this flag. Does device need it? Does device really
care that guest can't recover?
quoted hunk
---
 drivers/net/virtio_net.c           | 2 ++
 include/uapi/linux/virtio_config.h | 3 +++
 2 files changed, 5 insertions(+)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 5e349226f7c1..15483a982106 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -2801,12 +2801,14 @@ static struct virtio_device_id id_table[] = {
 
 static unsigned int features[] = {
 	VIRTNET_FEATURES,
+	VIRTIO_F_CAN_RESET,
 };
 
 static unsigned int features_legacy[] = {
 	VIRTNET_FEATURES,
 	VIRTIO_NET_F_GSO,
 	VIRTIO_F_ANY_LAYOUT,
+	VIRTIO_F_CAN_RESET,
 };
 
 static struct virtio_driver virtio_net_driver = {
diff --git a/include/uapi/linux/virtio_config.h b/include/uapi/linux/virtio_config.h
index 308e2096291f..726be44a04d3 100644
--- a/include/uapi/linux/virtio_config.h
+++ b/include/uapi/linux/virtio_config.h
@@ -56,6 +56,9 @@
  * suppressed them? */
 #define VIRTIO_F_NOTIFY_ON_EMPTY	24
 
+/* Driver supports the VIRTIO_CONFIG_S_NEEDS_RESET command */
+#define VIRTIO_F_CAN_RESET		25
+
 /* Can the device handle any descriptor layout? */
 #define VIRTIO_F_ANY_LAYOUT		27
 #endif /* VIRTIO_CONFIG_NO_LEGACY */
-- 
2.14.1.342.g6490525c54-goog

Willem de Bruijn replied to Michael S. Tsirkin

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On Tue, Aug 29, 2017 at 4:16 PM, Michael S. Tsirkin [off-list ref] wrote:
On Tue, Aug 29, 2017 at 04:07:59PM -0400, Willem de Bruijn wrote:
quoted
From: Willem de Bruijn <willemb@google.com>

Implement a mechanism to signal that a virtio device implements the
VIRTIO_CONFIG_S_NEEDS_RESET command.

Testing for VIRTIO_CONFIG_S_NEEDS_RESET support by issuing the request
and verifying the reset state would require an expensive state change.

To avoid that, add a status bit to the feature register used during
feature negotiation between host and guest.

Set the bit for virtio-net, which supports the feature.

Signed-off-by: Willem de Bruijn <willemb@google.com>
All virtio 1 devices have the reset feature
I don't quite follow. No device drivers implement this request currently?

so maybe guest does
not need this flag. Does device need it? Does device really
care that guest can't recover?
If all device drivers support it, then the flag is not needed.

But as long as legacy device drivers can exist that do not support
this feature, it has to have a way of differentiating between the two.
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