| Message ID | 1458075853-14789-2-git-send-email-aconole@redhat.com |
|---|---|
| State | RFC, archived |
| Delegated to: | David Miller |
| Headers | show |
On Tue, 15 Mar 2016 17:04:12 -0400 Aaron Conole <aconole@redhat.com> wrote: > --- a/include/uapi/linux/virtio_net.h > +++ b/include/uapi/linux/virtio_net.h > @@ -55,6 +55,7 @@ > #define VIRTIO_NET_F_MQ 22 /* Device supports Receive Flow > * Steering */ > #define VIRTIO_NET_F_CTRL_MAC_ADDR 23 /* Set MAC address */ > +#define VIRTIO_NET_F_MTU 25 /* Device supports Default MTU Negotiation */ > > #ifndef VIRTIO_NET_NO_LEGACY > #define VIRTIO_NET_F_GSO 6 /* Host handles pkts w/ any GSO type */ > @@ -73,6 +74,8 @@ struct virtio_net_config { > * Legal values are between 1 and 0x8000 > */ > __u16 max_virtqueue_pairs; > + /* Default maximum transmit unit advice */ > + __u16 mtu; > } __attribute__((packed)); > > /* You can't change user visible headers without breaking ABI. This structure might be used by other user code. Also how can this work if host is using old size of structure.
On Wed, Mar 16, 2016 at 11:23:14AM -0700, Stephen Hemminger wrote: > On Tue, 15 Mar 2016 17:04:12 -0400 > Aaron Conole <aconole@redhat.com> wrote: > > > --- a/include/uapi/linux/virtio_net.h > > +++ b/include/uapi/linux/virtio_net.h > > @@ -55,6 +55,7 @@ > > #define VIRTIO_NET_F_MQ 22 /* Device supports Receive Flow > > * Steering */ > > #define VIRTIO_NET_F_CTRL_MAC_ADDR 23 /* Set MAC address */ > > +#define VIRTIO_NET_F_MTU 25 /* Device supports Default MTU Negotiation */ > > > > #ifndef VIRTIO_NET_NO_LEGACY > > #define VIRTIO_NET_F_GSO 6 /* Host handles pkts w/ any GSO type */ > > @@ -73,6 +74,8 @@ struct virtio_net_config { > > * Legal values are between 1 and 0x8000 > > */ > > __u16 max_virtqueue_pairs; > > + /* Default maximum transmit unit advice */ > > + __u16 mtu; > > } __attribute__((packed)); > > > > /* > > You can't change user visible headers without breaking ABI. > This structure might be used by other user code. Then this userspace is broken. If someone uses virtio code one has to follow virtio spec. And Virtio spec makes it very clear that 1. config space size can change at any time 2. fields can only be accessed if the correct feature bit has been both advertized in host bitmap and acknowledged in guest bitmap > Also how can this > work if host is using old size of structure. It works because access is guarded by feature bit check.
Stephen Hemminger <stephen@networkplumber.org> writes: > On Tue, 15 Mar 2016 17:04:12 -0400 > Aaron Conole <aconole@redhat.com> wrote: > >> --- a/include/uapi/linux/virtio_net.h >> +++ b/include/uapi/linux/virtio_net.h >> @@ -55,6 +55,7 @@ >> #define VIRTIO_NET_F_MQ 22 /* Device supports Receive Flow >> * Steering */ >> #define VIRTIO_NET_F_CTRL_MAC_ADDR 23 /* Set MAC address */ >> +#define VIRTIO_NET_F_MTU 25 /* Device supports Default MTU Negotiation */ >> >> #ifndef VIRTIO_NET_NO_LEGACY >> #define VIRTIO_NET_F_GSO 6 /* Host handles pkts w/ any GSO type */ >> @@ -73,6 +74,8 @@ struct virtio_net_config { >> * Legal values are between 1 and 0x8000 >> */ >> __u16 max_virtqueue_pairs; >> + /* Default maximum transmit unit advice */ >> + __u16 mtu; >> } __attribute__((packed)); >> >> /* > > You can't change user visible headers without breaking ABI. > This structure might be used by other user code. Also how can this > work if host is using old size of structure. How else can this field be added and remain compliant with the spec? The spec requires that mtu be passed in the virtio_net_config field. As for old sizeof, I think the absence of the VIRTIO_NET_F_MTU bit being asserted is confirmation that mtu is not valid (at least, it is implied in the spec). Thanks so much for the review, Stephen! -Aaron
On Thu, 17 Mar 2016 17:10:55 -0400 Aaron Conole <aconole@redhat.com> wrote: > Stephen Hemminger <stephen@networkplumber.org> writes: > > > On Tue, 15 Mar 2016 17:04:12 -0400 > > Aaron Conole <aconole@redhat.com> wrote: > > > >> --- a/include/uapi/linux/virtio_net.h > >> +++ b/include/uapi/linux/virtio_net.h > >> @@ -55,6 +55,7 @@ > >> #define VIRTIO_NET_F_MQ 22 /* Device supports Receive Flow > >> * Steering */ > >> #define VIRTIO_NET_F_CTRL_MAC_ADDR 23 /* Set MAC address */ > >> +#define VIRTIO_NET_F_MTU 25 /* Device supports Default MTU Negotiation */ > >> > >> #ifndef VIRTIO_NET_NO_LEGACY > >> #define VIRTIO_NET_F_GSO 6 /* Host handles pkts w/ any GSO type */ > >> @@ -73,6 +74,8 @@ struct virtio_net_config { > >> * Legal values are between 1 and 0x8000 > >> */ > >> __u16 max_virtqueue_pairs; > >> + /* Default maximum transmit unit advice */ > >> + __u16 mtu; > >> } __attribute__((packed)); > >> > >> /* > > > > You can't change user visible headers without breaking ABI. > > This structure might be used by other user code. Also how can this > > work if host is using old size of structure. > > How else can this field be added and remain compliant with the spec? The > spec requires that mtu be passed in the virtio_net_config field. > > As for old sizeof, I think the absence of the VIRTIO_NET_F_MTU bit being > asserted is confirmation that mtu is not valid (at least, it is implied > in the spec). Michael is right as long as the code checks for MTU flag before referencing the mtu field, everything is fine. Actually, the structure is never used directly only by fetching fields with offsetof
diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h index ec32293..41a6a01 100644 --- a/include/uapi/linux/virtio_net.h +++ b/include/uapi/linux/virtio_net.h @@ -55,6 +55,7 @@ #define VIRTIO_NET_F_MQ 22 /* Device supports Receive Flow * Steering */ #define VIRTIO_NET_F_CTRL_MAC_ADDR 23 /* Set MAC address */ +#define VIRTIO_NET_F_MTU 25 /* Device supports Default MTU Negotiation */ #ifndef VIRTIO_NET_NO_LEGACY #define VIRTIO_NET_F_GSO 6 /* Host handles pkts w/ any GSO type */ @@ -73,6 +74,8 @@ struct virtio_net_config { * Legal values are between 1 and 0x8000 */ __u16 max_virtqueue_pairs; + /* Default maximum transmit unit advice */ + __u16 mtu; } __attribute__((packed)); /*
This commit adds the feature bit and associated mtu device entry for the virtio network device. Future commits will make use of these bits to support negotiated MTU. Signed-off-by: Aaron Conole <aconole@bytheb.org> --- v2: * No change include/uapi/linux/virtio_net.h | 3 +++ 1 file changed, 3 insertions(+)