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SubjectRe: [PATCH 3/5] virtio: Don't expose legacy net features when VIRTIO_NET_NO_LEGACY defined.
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Rusty Russell <rusty@rustcorp.com.au> writes:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
>> On Fri, Feb 06, 2015 at 03:36:54PM +1030, Rusty Russell wrote:
>>> In particular, the virtio header always has the u16 num_buffers field.
>>> We define a new 'struct virtio_net_modern_hdr' for this (rather than
>>> simply calling it 'struct virtio_net_hdr', to avoid nasty type errors
>>> if some parts of a project define VIRTIO_NET_NO_LEGACY and some don't.
>>>
>>> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
>>> ---
>>> include/uapi/linux/virtio_net.h | 30 ++++++++++++++++++++++++++++--
>>> 1 file changed, 28 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
>>> index b5f1677b291c..32754f3000e8 100644
>>> --- a/include/uapi/linux/virtio_net.h
>>> +++ b/include/uapi/linux/virtio_net.h
>>> @@ -35,7 +35,6 @@
>>> #define VIRTIO_NET_F_CSUM 0 /* Host handles pkts w/ partial csum */
>>> #define VIRTIO_NET_F_GUEST_CSUM 1 /* Guest handles pkts w/ partial csum */
>>> #define VIRTIO_NET_F_MAC 5 /* Host has given MAC address. */
>>> -#define VIRTIO_NET_F_GSO 6 /* Host handles pkts w/ any GSO type */
>>> #define VIRTIO_NET_F_GUEST_TSO4 7 /* Guest can handle TSOv4 in. */
>>> #define VIRTIO_NET_F_GUEST_TSO6 8 /* Guest can handle TSOv6 in. */
>>> #define VIRTIO_NET_F_GUEST_ECN 9 /* Guest can handle TSO[6] w/ ECN in. */
>>> @@ -56,6 +55,10 @@
>>> * Steering */
>>> #define VIRTIO_NET_F_CTRL_MAC_ADDR 23 /* Set MAC address */
>>>
>>> +#ifndef VIRTIO_NET_NO_LEGACY
>>> +#define VIRTIO_NET_F_GSO 6 /* Host handles pkts w/ any GSO type */
>>> +#endif /* VIRTIO_NET_NO_LEGACY */
>>> +
>>> #define VIRTIO_NET_S_LINK_UP 1 /* Link is up */
>>> #define VIRTIO_NET_S_ANNOUNCE 2 /* Announcement is needed */
>>>
>>> @@ -71,8 +74,9 @@ struct virtio_net_config {
>>> __u16 max_virtqueue_pairs;
>>> } __attribute__((packed));
>>>
>>> +#ifndef VIRTIO_NET_NO_LEGACY
>>> /* This header comes first in the scatter-gather list.
>>> - * If VIRTIO_F_ANY_LAYOUT is not negotiated, it must
>>> + * For legacy virtio, if VIRTIO_F_ANY_LAYOUT is not negotiated, it must
>>> * be the first element of the scatter-gather list. If you don't
>>> * specify GSO or CSUM features, you can simply ignore the header. */
>>> struct virtio_net_hdr {
>>> @@ -97,6 +101,28 @@ struct virtio_net_hdr_mrg_rxbuf {
>>> struct virtio_net_hdr hdr;
>>> __virtio16 num_buffers; /* Number of merged rx buffers */
>>> };
>>> +#else /* ... VIRTIO_NET_NO_LEGACY */
>>> +/*
>>> + * This header comes first in the scatter-gather list. If you don't
>>> + * specify GSO or CSUM features, you can simply ignore the header.
>>> + */
>>> +struct virtio_net_modern_hdr {
>>> +#define VIRTIO_NET_HDR_F_NEEDS_CSUM 1 /* Use csum_start, csum_offset */
>>> +#define VIRTIO_NET_HDR_F_DATA_VALID 2 /* Csum is valid */
>>> + __u8 flags;
>>> +#define VIRTIO_NET_HDR_GSO_NONE 0 /* Not a GSO frame */
>>> +#define VIRTIO_NET_HDR_GSO_TCPV4 1 /* GSO frame, IPv4 TCP (TSO) */
>>> +#define VIRTIO_NET_HDR_GSO_UDP 3 /* GSO frame, IPv4 UDP (UFO) */
>>> +#define VIRTIO_NET_HDR_GSO_TCPV6 4 /* GSO frame, IPv6 TCP */
>>> +#define VIRTIO_NET_HDR_GSO_ECN 0x80 /* TCP has ECN set */
>>> + __u8 gso_type;
>>> + __virtio16 hdr_len; /* Ethernet + IP + tcp/udp hdrs */
>>> + __virtio16 gso_size; /* Bytes to append to hdr_len per frame */
>>> + __virtio16 csum_start; /* Position to start checksumming from */
>>> + __virtio16 csum_offset; /* Offset after that to place checksum */
>>> + __virtio16 num_buffers; /* Number of merged rx buffers */
>>> +};
>>> +#endif /* ...VIRTIO_NET_NO_LEGACY */
>>
>> This kind of masks the fact that it's the same as
>> virtio_net_hdr_mrg_rxbuf. So it's forcing people to duplicate
>> code for transitional devices.
>
> It's a hard problem to solve; someone suffers. My preference is to make
> non-legacy implementations as "clean" as possible, even if transitional
> devices suffer (since their suffering should be shorter-lived).
>
> Transitional devices can't define VIRTIO_NET_NO_LEGACY, so they'll
> have to use virtio_net_hdr_mrg_rxbuf anyway.

Here's what I ended up with. In effect, it's just the name change
and a comment that it's the same as the legacy virtio_net_hdr_mrg_rxbuf:


Subject: virtio: Don't expose legacy net features when VIRTIO_NET_NO_LEGACY defined.

In particular, the virtio header always has the u16 num_buffers field.
We define a new 'struct virtio_net_hdr_v1' for this (rather than
simply calling it 'struct virtio_net_hdr', to avoid nasty type errors
if some parts of a project define VIRTIO_NET_NO_LEGACY and some don't.

Transitional devices (which can't define VIRTIO_NET_NO_LEGACY) will
have to keep using struct virtio_net_hdr_mrg_rxbuf, which has the same
byte layout as struct virtio_net_hdr_v1.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
index b5f1677b291c..4a9b58113d6e 100644
--- a/include/uapi/linux/virtio_net.h
+++ b/include/uapi/linux/virtio_net.h
@@ -35,7 +35,6 @@
#define VIRTIO_NET_F_CSUM 0 /* Host handles pkts w/ partial csum */
#define VIRTIO_NET_F_GUEST_CSUM 1 /* Guest handles pkts w/ partial csum */
#define VIRTIO_NET_F_MAC 5 /* Host has given MAC address. */
-#define VIRTIO_NET_F_GSO 6 /* Host handles pkts w/ any GSO type */
#define VIRTIO_NET_F_GUEST_TSO4 7 /* Guest can handle TSOv4 in. */
#define VIRTIO_NET_F_GUEST_TSO6 8 /* Guest can handle TSOv6 in. */
#define VIRTIO_NET_F_GUEST_ECN 9 /* Guest can handle TSO[6] w/ ECN in. */
@@ -56,6 +55,10 @@
* Steering */
#define VIRTIO_NET_F_CTRL_MAC_ADDR 23 /* Set MAC address */

+#ifndef VIRTIO_NET_NO_LEGACY
+#define VIRTIO_NET_F_GSO 6 /* Host handles pkts w/ any GSO type */
+#endif /* VIRTIO_NET_NO_LEGACY */
+
#define VIRTIO_NET_S_LINK_UP 1 /* Link is up */
#define VIRTIO_NET_S_ANNOUNCE 2 /* Announcement is needed */

@@ -71,8 +74,9 @@ struct virtio_net_config {
__u16 max_virtqueue_pairs;
} __attribute__((packed));

+#ifndef VIRTIO_NET_NO_LEGACY
/* This header comes first in the scatter-gather list.
- * If VIRTIO_F_ANY_LAYOUT is not negotiated, it must
+ * For legacy virtio, if VIRTIO_F_ANY_LAYOUT is not negotiated, it must
* be the first element of the scatter-gather list. If you don't
* specify GSO or CSUM features, you can simply ignore the header. */
struct virtio_net_hdr {
@@ -97,6 +101,30 @@ struct virtio_net_hdr_mrg_rxbuf {
struct virtio_net_hdr hdr;
__virtio16 num_buffers; /* Number of merged rx buffers */
};
+#else /* ... VIRTIO_NET_NO_LEGACY */
+/*
+ * This header comes first in the scatter-gather list. If you don't
+ * specify GSO or CSUM features, you can simply ignore the header.
+ *
+ * This is bitwise-equivalent to the legacy struct virtio_net_hdr_mrg_rxbuf.
+ */
+struct virtio_net_hdr_v1 {
+#define VIRTIO_NET_HDR_F_NEEDS_CSUM 1 /* Use csum_start, csum_offset */
+#define VIRTIO_NET_HDR_F_DATA_VALID 2 /* Csum is valid */
+ __u8 flags;
+#define VIRTIO_NET_HDR_GSO_NONE 0 /* Not a GSO frame */
+#define VIRTIO_NET_HDR_GSO_TCPV4 1 /* GSO frame, IPv4 TCP (TSO) */
+#define VIRTIO_NET_HDR_GSO_UDP 3 /* GSO frame, IPv4 UDP (UFO) */
+#define VIRTIO_NET_HDR_GSO_TCPV6 4 /* GSO frame, IPv6 TCP */
+#define VIRTIO_NET_HDR_GSO_ECN 0x80 /* TCP has ECN set */
+ __u8 gso_type;
+ __virtio16 hdr_len; /* Ethernet + IP + tcp/udp hdrs */
+ __virtio16 gso_size; /* Bytes to append to hdr_len per frame */
+ __virtio16 csum_start; /* Position to start checksumming from */
+ __virtio16 csum_offset; /* Offset after that to place checksum */
+ __virtio16 num_buffers; /* Number of merged rx buffers */
+};
+#endif /* ...VIRTIO_NET_NO_LEGACY */

/*
* Control virtqueue data structures

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