On Fri, May 26, 2017 at 02:04:23PM -0700, Joe Stringer wrote:
> On 25 May 2017 at 13:52, Ben Pfaff <b...@ovn.org> wrote:
> > I know of two reasons to mark a structure as "packed". The first is
> > because the structure must match some defined interface and therefore
> > compiler-inserted padding between or after members would cause its layout
> > to diverge from that interface. This is not a problem in a structure that
> > follows the general alignment rules that are seen in ABIs for all the
> > architectures that OVS cares about: basically, that a struct member needs
> > to be aligned on a boundary that is a multiple of the member's size.
> >
> > The second reason is because instances of the struct tend to be at
> > misaligned addresses.
> >
> > struct eth_header and struct vlan_eth_header are normally aligned on
> > 16-bit boundaries (at least), and they contain only 16-bit members, so
> > there's no need to pack them. This commit removes the packed annotation.
> >
> > This commit also removes the packed annotation from struct llc_header.
> > Since that struct only contains 8-bit members, I don't know of any benefit
> > to packing it, period.
> >
> > This commit also removes a few more packed annotations that are much less
> > important.
> >
> > When these packed annotations were removed, it caused a few warnings
> > related to casts from 'uint8_t *' to more strictly aligned pointer types,
> > related to struct ovs_action_push_tnl. That's because that struct had a
> > trailing member used to store packet headers, that was declared as
> > a uint8_t[]. Before, when this was cast to 'struct eth_header *', there
> > was no change in alignment since eth_header was packed; now that
> > eth_header is not packed, the compiler considers it suspicious. This
> > commit avoids that problem by changing the member from uint8_t[] to
> > uint32_t[], which assures the compiler that it is properly aligned.
> >
> > CC: Joe Stringer <j...@ovn.org>
> > Signed-off-by: Ben Pfaff <b...@ovn.org>
> > ---
>
> Thanks for the patch, this fixes the primary issue I observed and
> looking at the "pahole" output on vswitchd compiled by clang, it looks
> mostly correct on my x86-64 dev box. Assuming you're not concerned by
> the below feedback:
> Acked-by: Joe Stringer <j...@ovn.org>
Thanks.
> > datapath/linux/compat/include/linux/openvswitch.h | 2 +-
> > lib/packets.h | 9 +++------
> > lib/stp.c | 8 +++-----
> > ofproto/bundles.h | 4 ++--
> > 4 files changed, 9 insertions(+), 14 deletions(-)
> >
> > diff --git a/datapath/linux/compat/include/linux/openvswitch.h
> > b/datapath/linux/compat/include/linux/openvswitch.h
> > index d22102e224a7..55ec6c13fbd2 100644
> > --- a/datapath/linux/compat/include/linux/openvswitch.h
> > +++ b/datapath/linux/compat/include/linux/openvswitch.h
> > @@ -715,7 +715,7 @@ struct ovs_action_push_tnl {
> > uint32_t out_port;
> > uint32_t header_len;
> > uint32_t tnl_type; /* For logging. */
> > - uint8_t header[TNL_PUSH_HEADER_SIZE];
> > + uint32_t header[TNL_PUSH_HEADER_SIZE / 4];
> > };
> > #endif
> >
>
> Would you mind submitting this hunk upstream to net-next?
It's inside #ifndef __KERNEL__.
> > -enum OVS_PACKED_ENUM bundle_state {
> > +enum bundle_state {
> > BS_OPEN,
> > BS_CLOSED
> > };
>
> This hunk actually appears to influence the layout of "struct
> ofp_bundle" which increases the size by 8 bytes on my system. That
> said, it still appears to be 2 cachelines long in both cases and I
> don't know whether we are specifically trying to tailor the size of
> this structure (eg, for performance reasons).
I assumed that this was just cargo-culting from some other use of
OVS_PACKED_ENUM. I don't see how OpenFlow bundles are performance or
size sensitive.
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