Hi Samudrala, On Fri, Mar 26, 2021 at 11:45 PM Samudrala, Sridhar <sridhar.samudr...@intel.com> wrote: > On 3/26/2021 1:06 AM, Geert Uytterhoeven wrote: > > On Thu, Mar 25, 2021 at 11:29 PM Tony Nguyen <anthony.l.ngu...@intel.com> > > wrote: > > From: Norbert Ciosek <norbertx.cio...@intel.com> > > > > Remove padding from RSS structures. Previous layout > > could lead to unwanted compiler optimizations > > in loops when iterating over key and lut arrays. > > > > From an earlier private conversation with Mateusz, I understand the real > > explanation is that key[] and lut[] must be at the end of the > > structures, because they are used as flexible array members? > > > > Fixes: 65ece6de0114 ("virtchnl: Add missing explicit padding to structures") > > Signed-off-by: Norbert Ciosek <norbertx.cio...@intel.com> > > Tested-by: Konrad Jankowski <konrad0.jankow...@intel.com> > > Signed-off-by: Tony Nguyen <anthony.l.ngu...@intel.com> > > > > --- a/include/linux/avf/virtchnl.h > > +++ b/include/linux/avf/virtchnl.h > > @@ -476,7 +476,6 @@ struct virtchnl_rss_key { > > u16 vsi_id; > > u16 key_len; > > u8 key[1]; /* RSS hash key, packed bytes */ > > - u8 pad[1]; > > }; > > > > VIRTCHNL_CHECK_STRUCT_LEN(6, virtchnl_rss_key); > > @@ -485,7 +484,6 @@ struct virtchnl_rss_lut { > > u16 vsi_id; > > u16 lut_entries; > > u8 lut[1]; /* RSS lookup table */ > > - u8 pad[1]; > > }; > > > > If you use a flexible array member, it should be declared without a size, > > i.e. > > > > u8 key[]; > > > > Everything else is (trying to) fool the compiler, and leading to undefined > > behavior, and people (re)adding explicit padding. > > This header file is shared across other OSes that use C++ that doesn't support > flexible arrays. So the structures in this file use an array of size 1 as a > last > element to enable variable sized arrays.
I don't think it is accepted practice to have non-Linux-isms in include/*linux*/avf/virtchnl.h header files. Moreover, using a size of 1 is counter-intuitive for people used to Linux kernel development, and may lead to off-by-one errors in calculation of sizes. If you insist on ignoring the above, this definitely deserves a comment next to the member's declaration. Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- ge...@linux-m68k.org In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds