On Thu, 24 Sep 2015 12:50:59 +0300
"Michael S. Tsirkin" <m...@redhat.com> wrote:

> On Thu, Sep 24, 2015 at 09:25:45AM +0200, Greg Kurz wrote:
> > On Wed, 23 Sep 2015 19:45:08 +0100
> > David Woodhouse <dw...@infradead.org> wrote:
> > 
> > > Commit 7d82410950aa ("virtio: add explicit big-endian support to memory
> > > accessors") accidentally changed the virtio_net header used by
> > > AF_PACKET with PACKET_VNET_HDR from host-endian to big-endian.
> > > 
> > 
> > Hi David,
> > 
> > Oops my bad... I obviously overlooked this one when adding cross-endian
> > support.
> > 
> > > Since virtio_legacy_is_little_endian() is a very long identifier,
> > > define a VIO_LE macro and use that throughout the code instead of the
> > 
> > VIO usually refers to virtual IO adapters for the PowerPC pSeries platform.
> > 
> > > hard-coded 'false' for little-endian.
> > > 
> > 
> > What about introducing dedicated accessors as it is done in many other
> > locations where we do virtio byteswap ? Something like:
> > 
> > static inline bool packet_is_little_endian(void)
> > {
> >     return virtio_legacy_is_little_endian();
> > }
> > 
> > static inline u16 packet16_to_cpu(__virtio16 val)
> > {
> >     return __virtio16_to_cpu(packet_is_little_endian(), val);
> > }
> > 
> > static inline __virtio16 cpu_to_packet16(u16 val)
> > {
> >     return __cpu_to_virtio16(packet_is_little_endian(), val);
> > }
> > 
> > It results in prettier code IMHO. Have a look at drivers/net/tun.c or
> > drivers/vhost/vhost.c.
> > 
> > > This restores the ABI to match 4.1 and earlier kernels, and makes my
> > > test program work again.
> > > 
> > 
> > BTW, there is still work to do if we want to support cross-endian legacy or
> > virtio 1 on a big endian arch...
> > 
> > Cheers.
> > 
> > --
> > Greg
> 
> It seems the API that we have is a confusing one.
> 
> virtio endian-ness is either native or little, depending on a flag, so
> __virtio16_to_cpu seems to mean "either native to cpu or little to cpu
> depending on flag".
> 
> It used to be like that, but not anymore.
> 
> This leads to all kind of bugs.
> 
> For example, I have only now realized vhost_is_little_endian isn't a
> constant on LE hosts if cross-endian support is not compiled.
> 
> I think we need to fix it, but also think about a better API.
> 

Your original API is good and works well for all the callers that
don't care for cross-endian support.

I guess we'd rather move the cross-endian burden to the few callers who
need it, i.e. tun, macvtap and vhost when cross-endian is compiled.

More or less like this:

/* Original API : either little-endian or native */
static inline u16 __virtio16_to_cpu(bool little_endian, __virtio16 val)
{
        if (little_endian)
                return le16_to_cpu((__force __le16)val);
        else
                return (__force u16)val;
}

#ifdef CONFIG_VHOST_CROSS_ENDIAN_LEGACY

static inline u16 vhost16_to_cpu(struct vhost_virtqueue *vq, __virtio16 val)
{
        /* little-endian because of virtio 1 */
        if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
                return __virtio16_to_cpu(true, val);

#ifdef __LITTLE_ENDIAN
        /* native little-endian */
        return (__force u16)val;
#else
        /* native big-endian */
        return be16_to_cpu((__force __be16)val);
#endif
}

#else

static inline u16 vhost16_to_cpu(struct vhost_virtqueue *vq, __virtio16 val)
{
#ifdef __LITTLE_ENDIAN
        /* fast path for little-endian host */
        return __virtio16_to_cpu(true, val);
#else
        /* slow path for big-endian host */
        return __virtio16_to_cpu(vhost_has_feature(vq, VIRTIO_F_VERSION_1), 
val);
#endif
}

#endif

Does it make sense ?

> 
> > > Signed-off-by: David Woodhouse <david.woodho...@intel.com>
> > > ---
> > > On Wed, 2015-09-23 at 11:09 -0700, David Miller wrote:
> > > > > +#define VIO_LE virtio_legacy_is_little_endian()
> > > > 
> > > > When you define a shorthand macro, the defines to a function call,
> > > > make the macro have parenthesis too.
> > > 
> > > In which case I suppose it also wants to be lower-case. Although
> > > "function call" is a bit strong since it's effectively just a constant.
> > > I'm still wondering if it'd be nicer just to use (__force u16) instead.
> > > 
> > > diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> > > index 7b8e39a..aa4b15c 100644
> > > --- a/net/packet/af_packet.c
> > > +++ b/net/packet/af_packet.c
> > > @@ -230,6 +230,8 @@ struct packet_skb_cb {
> > >   } sa;
> > >  };
> > >  
> > > +#define vio_le() virtio_legacy_is_little_endian()
> > > +
> > >  #define PACKET_SKB_CB(__skb)     ((struct packet_skb_cb *)((__skb)->cb))
> > >  
> > >  #define GET_PBDQC_FROM_RB(x)     ((struct tpacket_kbdq_core 
> > > *)(&(x)->prb_bdqc))
> > > @@ -2680,15 +2682,15 @@ static int packet_snd(struct socket *sock, struct 
> > > msghdr *msg, size_t len)
> > >                   goto out_unlock;
> > >  
> > >           if ((vnet_hdr.flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) &&
> > > -             (__virtio16_to_cpu(false, vnet_hdr.csum_start) +
> > > -              __virtio16_to_cpu(false, vnet_hdr.csum_offset) + 2 >
> > > -               __virtio16_to_cpu(false, vnet_hdr.hdr_len)))
> > > -                 vnet_hdr.hdr_len = __cpu_to_virtio16(false,
> > > -                          __virtio16_to_cpu(false, vnet_hdr.csum_start) +
> > > -                         __virtio16_to_cpu(false, vnet_hdr.csum_offset) 
> > > + 2);
> > > +             (__virtio16_to_cpu(vio_le(), vnet_hdr.csum_start) +
> > > +              __virtio16_to_cpu(vio_le(), vnet_hdr.csum_offset) + 2 >
> > > +               __virtio16_to_cpu(vio_le(), vnet_hdr.hdr_len))) actually
> > > +                 vnet_hdr.hdr_len = __cpu_to_virtio16(vio_le(),
> > > +                          __virtio16_to_cpu(vio_le(), 
> > > vnet_hdr.csum_start) +
> > > +                         __virtio16_to_cpu(vio_le(), 
> > > vnet_hdr.csum_offset) + 2);
> > >  
> > >           err = -EINVAL;
> > > -         if (__virtio16_to_cpu(false, vnet_hdr.hdr_len) > len)
> > > +         if (__virtio16_to_cpu(vio_le(), vnet_hdr.hdr_len) > len)
> > >                   goto out_unlock;
> > >  
> > >           if (vnet_hdr.gso_type != VIRTIO_NET_HDR_GSO_NONE) {
> > > @@ -2731,7 +2733,7 @@ static int packet_snd(struct socket *sock, struct 
> > > msghdr *msg, size_t len)
> > >   hlen = LL_RESERVED_SPACE(dev);
> > >   tlen = dev->needed_tailroom;
> > >   skb = packet_alloc_skb(sk, hlen + tlen, hlen, len,
> > > -                        __virtio16_to_cpu(false, vnet_hdr.hdr_len),
> > > +                        __virtio16_to_cpu(vio_le(), vnet_hdr.hdr_len),
> > >                          msg->msg_flags & MSG_DONTWAIT, &err);
> > >   if (skb == NULL)
> > >           goto out_unlock;
> > > @@ -2778,8 +2780,8 @@ static int packet_snd(struct socket *sock, struct 
> > > msghdr *msg, size_t len)
> > >  
> > >   if (po->has_vnet_hdr) {
> > >           if (vnet_hdr.flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) {
> > > -                 u16 s = __virtio16_to_cpu(false, vnet_hdr.csum_start);
> > > -                 u16 o = __virtio16_to_cpu(false, vnet_hdr.csum_offset);
> > > +                 u16 s = __virtio16_to_cpu(vio_le(), 
> > > vnet_hdr.csum_start);
> > > +                 u16 o = __virtio16_to_cpu(vio_le(), 
> > > vnet_hdr.csum_offset);
> > >                   if (!skb_partial_csum_set(skb, s, o)) {
> > >                           err = -EINVAL;
> > >                           goto out_free;
> > > @@ -2787,7 +2789,7 @@ static int packet_snd(struct socket *sock, struct 
> > > msghdr *msg, size_t len)
> > >           }
> > >  
> > >           skb_shinfo(skb)->gso_size =
> > > -                 __virtio16_to_cpu(false, vnet_hdr.gso_size);
> > > +                 __virtio16_to_cpu(vio_le(), vnet_hdr.gso_size);
> > >           skb_shinfo(skb)->gso_type = gso_type;
> > >  
> > >           /* Header must be checked, and gso_segs computed. */
> > > @@ -3161,9 +3163,9 @@ static int packet_recvmsg(struct socket *sock, 
> > > struct msghdr *msg, size_t len,
> > >  
> > >                   /* This is a hint as to how much should be linear. */
> > >                   vnet_hdr.hdr_len =
> > > -                         __cpu_to_virtio16(false, skb_headlen(skb));
> > > +                         __cpu_to_virtio16(vio_le(), skb_headlen(skb));
> > >                   vnet_hdr.gso_size =
> > > -                         __cpu_to_virtio16(false, sinfo->gso_size);
> > > +                         __cpu_to_virtio16(vio_le(), sinfo->gso_size);
> > >                   if (sinfo->gso_type & SKB_GSO_TCPV4)
> > >                           vnet_hdr.gso_type = VIRTIO_NET_HDR_GSO_TCPV4;
> > >                   else if (sinfo->gso_type & SKB_GSO_TCPV6)
> > > @@ -3181,9 +3183,9 @@ static int packet_recvmsg(struct socket *sock, 
> > > struct msghdr *msg, size_t len,
> > >  
> > >           if (skb->ip_summed == CHECKSUM_PARTIAL) {
> > >                   vnet_hdr.flags = VIRTIO_NET_HDR_F_NEEDS_CSUM;
> > > -                 vnet_hdr.csum_start = __cpu_to_virtio16(false,
> > > +                 vnet_hdr.csum_start = __cpu_to_virtio16(vio_le(),
> > >                                     skb_checksum_start_offset(skb));
> > > -                 vnet_hdr.csum_offset = __cpu_to_virtio16(false,
> > > +                 vnet_hdr.csum_offset = __cpu_to_virtio16(vio_le(),
> > >                                                    skb->csum_offset);
> > >           } else if (skb->ip_summed == CHECKSUM_UNNECESSARY) {
> > >                   vnet_hdr.flags = VIRTIO_NET_HDR_F_DATA_VALID;
> > > 
> 



-- 
Gregory Kurz                                     kurzg...@fr.ibm.com
                                                 gk...@linux.vnet.ibm.com
Software Engineer @ IBM/LTC                      http://www.ibm.com
Tel 33-5-6218-1607

"Anarchy is about taking complete responsibility for yourself."
        Alan Moore.

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