Stephen Hemminger
2016-Mar-16 18:23 UTC
[RFC v2 -next 1/2] virtio: Start feature MTU support
On Tue, 15 Mar 2016 17:04:12 -0400 Aaron Conole <aconole at 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.
Michael S. Tsirkin
2016-Mar-16 18:29 UTC
[RFC v2 -next 1/2] virtio: Start feature MTU support
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 at 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. -- MST
Stephen Hemminger <stephen at networkplumber.org> writes:> On Tue, 15 Mar 2016 17:04:12 -0400 > Aaron Conole <aconole at 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
Stephen Hemminger
2016-Mar-17 22:04 UTC
[RFC v2 -next 1/2] virtio: Start feature MTU support
On Thu, 17 Mar 2016 17:10:55 -0400 Aaron Conole <aconole at redhat.com> wrote:> Stephen Hemminger <stephen at networkplumber.org> writes: > > > On Tue, 15 Mar 2016 17:04:12 -0400 > > Aaron Conole <aconole at 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
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- [RFC v2 -next 1/2] virtio: Start feature MTU support